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Expected Values

Probability and Statistics · FE Reference Handbook section

Probability and Statistics
15 formulas
10 exam-style examples
~60 min
All Probability and Statistics lectures

Handbook notes for this section

Definitions and conditions exactly as the handbook states them.

  • Let X be a discrete random variable having a probability
  • Engineering Probability and Statistics
  • The expected value of X is defined as
  • The mean or expected value of the random variable X is now defined as
  • while the variance is given by
  • The standard deviation is given by

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1
Expected value of a discrete random variable — solve for expected value — Expected Values

An engineer computes the expected value of a warranty payout distribution. Given outcome 1 (x1) = 8.0000; probability 1 (p1) = 0.4800; outcome 2 (x2) = 14.5000, determine the expected value (EX).

Given

  • outcome1(x1)=8.0000outcome 1 (x_{1}) = 8.0000
  • probability1(p1)=0.4800probability 1 (p_{1}) = 0.4800
  • outcome2(x2)=14.5000outcome 2 (x_{2}) = 14.5000

Find

expected value (EX)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except EX is given, so isolate EX symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for EX:

    EX=x1p1+x2(1−p1)EX = x_1 p_1 + x_2(1-p_1)
  3. Step 3

    Listthegivens:outcome1(x1)=8.0000,probability1(p1)=0.4800,outcome2(x2)=14.5000List the givens: outcome 1 (x_{1}) = 8.0000, probability 1 (p_{1}) = 0.4800, outcome 2 (x_{2}) = 14.5000
  4. Step 4 — Substitute the given values:

    EX=x1p1+x2(1−p1)EX = x_1 p_1 + x_2(1-p_1)
  5. Step 5 — Evaluate:

    EX=11.3800EX = 11.3800
  6. Step 6 — Check: returning EX = 11.3800 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
EX=11.3800EX = 11.3800

Why the other options are there

  • 22.7600 — kept a factor of two that cancels in the correct rearrangement.
  • 5.6900 — dropped that same factor in the other direction.
  • 12.5180 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 2
Expected value of a discrete random variable — solve for outcome 2 — Expected Values (2)

A student finds the expected value of a two-outcome gamble. Given outcome 1 (x1) = 4.0000; probability 1 (p1) = 0.5000; expected value (EX) = 4.8800, determine the outcome 2 (x2).

Given

  • outcome1(x1)=4.0000outcome 1 (x_{1}) = 4.0000
  • probability1(p1)=0.5000probability 1 (p_{1}) = 0.5000
  • expectedvalue(EX)=4.8800expected value (EX) = 4.8800

Find

outcome 2 (x2)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except x2 is given, so isolate x2 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for x2:

    x2=E[X]−x1p11−p1x_{2} = \dfrac{E[X] - x_1 p_1}{1-p_1}
  3. Step 3

    Listthegivens:outcome1(x1)=4.0000,probability1(p1)=0.5000,expectedvalue(EX)=4.8800List the givens: outcome 1 (x_{1}) = 4.0000, probability 1 (p_{1}) = 0.5000, expected value (EX) = 4.8800
  4. Step 4 — Substitute the given values:

    x2=E[X]−x1p11−p1x_{2} = \dfrac{E[X] - x_1 p_1}{1-p_1}
  5. Step 5 — Evaluate:

    x2=5.7600x_{2} = 5.7600
  6. Step 6 — Check: returning x2 = 5.7600 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
x2=5.7600x_{2} = 5.7600

Why the other options are there

  • 11.5200 — kept a factor of two that cancels in the correct rearrangement.
  • 2.8800 — dropped that same factor in the other direction.
  • 6.3360 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 3
Expected value of a discrete random variable — solve for probability 1 — Expected Values (3)

The expected value of a production yield is estimated from probabilities. Given outcome 1 (x1) = 5.0000; outcome 2 (x2) = 19.0000; expected value (EX) = 8.9600, determine the probability 1 (p1).

Given

  • outcome1(x1)=5.0000outcome 1 (x_{1}) = 5.0000
  • outcome2(x2)=19.0000outcome 2 (x_{2}) = 19.0000
  • expectedvalue(EX)=8.9600expected value (EX) = 8.9600

Find

probability 1 (p1)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except p1 is given, so isolate p1 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for p1:

    p1=x2−E[X]x2−x1p_{1} = \dfrac{x_2 - E[X]}{x_2 - x_1}
  3. Step 3

    Listthegivens:outcome1(x1)=5.0000,outcome2(x2)=19.0000,expectedvalue(EX)=8.9600List the givens: outcome 1 (x_{1}) = 5.0000, outcome 2 (x_{2}) = 19.0000, expected value (EX) = 8.9600
  4. Step 4 — Substitute the given values:

    p1=x2−E[X]x2−x1p_{1} = \dfrac{x_2 - E[X]}{x_2 - x_1}
  5. Step 5 — Evaluate:

    p1=0.7171p_{1} = 0.7171
  6. Step 6 — Check: returning p1 = 0.7171 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
p1=0.7171p_{1} = 0.7171

Why the other options are there

  • 1.4343 — kept a factor of two that cancels in the correct rearrangement.
  • 0.3586 — dropped that same factor in the other direction.
  • 0.7889 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 4
Expected value of a discrete random variable — solve for expected value (case 2) — Expected Values (4)

An engineer computes the expected value of a warranty payout distribution. Given outcome 1 (x1) = 7.0000; probability 1 (p1) = 0.3800; outcome 2 (x2) = 11.0000, determine the expected value (EX).

Given

  • outcome1(x1)=7.0000outcome 1 (x_{1}) = 7.0000
  • probability1(p1)=0.3800probability 1 (p_{1}) = 0.3800
  • outcome2(x2)=11.0000outcome 2 (x_{2}) = 11.0000

Find

expected value (EX)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except EX is given, so isolate EX symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for EX:

    EX=x1p1+x2(1−p1)EX = x_1 p_1 + x_2(1-p_1)
  3. Step 3

    Listthegivens:outcome1(x1)=7.0000,probability1(p1)=0.3800,outcome2(x2)=11.0000List the givens: outcome 1 (x_{1}) = 7.0000, probability 1 (p_{1}) = 0.3800, outcome 2 (x_{2}) = 11.0000
  4. Step 4 — Substitute the given values:

    EX=x1p1+x2(1−p1)EX = x_1 p_1 + x_2(1-p_1)
  5. Step 5 — Evaluate:

    EX=9.4800EX = 9.4800
  6. Step 6 — Check: returning EX = 9.4800 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
EX=9.4800EX = 9.4800

Why the other options are there

  • 18.9600 — kept a factor of two that cancels in the correct rearrangement.
  • 4.7400 — dropped that same factor in the other direction.
  • 10.4280 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 5
Expected value of a discrete random variable — solve for outcome 2 (case 2) — Expected Values (5)

A student finds the expected value of a two-outcome gamble. Given outcome 1 (x1) = 10.0000; probability 1 (p1) = 0.4000; expected value (EX) = 8.3500, determine the outcome 2 (x2).

Given

  • outcome1(x1)=10.0000outcome 1 (x_{1}) = 10.0000
  • probability1(p1)=0.4000probability 1 (p_{1}) = 0.4000
  • expectedvalue(EX)=8.3500expected value (EX) = 8.3500

Find

outcome 2 (x2)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except x2 is given, so isolate x2 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for x2:

    x2=E[X]−x1p11−p1x_{2} = \dfrac{E[X] - x_1 p_1}{1-p_1}
  3. Step 3

    Listthegivens:outcome1(x1)=10.0000,probability1(p1)=0.4000,expectedvalue(EX)=8.3500List the givens: outcome 1 (x_{1}) = 10.0000, probability 1 (p_{1}) = 0.4000, expected value (EX) = 8.3500
  4. Step 4 — Substitute the given values:

    x2=E[X]−x1p11−p1x_{2} = \dfrac{E[X] - x_1 p_1}{1-p_1}
  5. Step 5 — Evaluate:

    x2=7.2500x_{2} = 7.2500
  6. Step 6 — Check: returning x2 = 7.2500 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
x2=7.2500x_{2} = 7.2500

Why the other options are there

  • 14.5000 — kept a factor of two that cancels in the correct rearrangement.
  • 3.6250 — dropped that same factor in the other direction.
  • 7.9750 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 6
Expected value of a discrete random variable — solve for probability 1 (case 2) — Expected Values (6)

The expected value of a production yield is estimated from probabilities. Given outcome 1 (x1) = 5.0000; outcome 2 (x2) = 17.5000; expected value (EX) = 2.8600, determine the probability 1 (p1).

Given

  • outcome1(x1)=5.0000outcome 1 (x_{1}) = 5.0000
  • outcome2(x2)=17.5000outcome 2 (x_{2}) = 17.5000
  • expectedvalue(EX)=2.8600expected value (EX) = 2.8600

Find

probability 1 (p1)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except p1 is given, so isolate p1 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for p1:

    p1=x2−E[X]x2−x1p_{1} = \dfrac{x_2 - E[X]}{x_2 - x_1}
  3. Step 3

    Listthegivens:outcome1(x1)=5.0000,outcome2(x2)=17.5000,expectedvalue(EX)=2.8600List the givens: outcome 1 (x_{1}) = 5.0000, outcome 2 (x_{2}) = 17.5000, expected value (EX) = 2.8600
  4. Step 4 — Substitute the given values:

    p1=x2−E[X]x2−x1p_{1} = \dfrac{x_2 - E[X]}{x_2 - x_1}
  5. Step 5 — Evaluate:

    p1=1.1712p_{1} = 1.1712
  6. Step 6 — Check: returning p1 = 1.1712 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
p1=1.1712p_{1} = 1.1712

Why the other options are there

  • 2.3424 — kept a factor of two that cancels in the correct rearrangement.
  • 0.5856 — dropped that same factor in the other direction.
  • 1.2883 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 7
Expected value of a discrete random variable — solve for expected value (case 3) — Expected Values (7)

An engineer computes the expected value of a warranty payout distribution. Given outcome 1 (x1) = 2.5000; probability 1 (p1) = 0.5500; outcome 2 (x2) = 12.0000, determine the expected value (EX).

Given

  • outcome1(x1)=2.5000outcome 1 (x_{1}) = 2.5000
  • probability1(p1)=0.5500probability 1 (p_{1}) = 0.5500
  • outcome2(x2)=12.0000outcome 2 (x_{2}) = 12.0000

Find

expected value (EX)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except EX is given, so isolate EX symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for EX:

    EX=x1p1+x2(1−p1)EX = x_1 p_1 + x_2(1-p_1)
  3. Step 3

    Listthegivens:outcome1(x1)=2.5000,probability1(p1)=0.5500,outcome2(x2)=12.0000List the givens: outcome 1 (x_{1}) = 2.5000, probability 1 (p_{1}) = 0.5500, outcome 2 (x_{2}) = 12.0000
  4. Step 4 — Substitute the given values:

    EX=x1p1+x2(1−p1)EX = x_1 p_1 + x_2(1-p_1)
  5. Step 5 — Evaluate:

    EX=6.7750EX = 6.7750
  6. Step 6 — Check: returning EX = 6.7750 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
EX=6.7750EX = 6.7750

Why the other options are there

  • 13.5500 — kept a factor of two that cancels in the correct rearrangement.
  • 3.3875 — dropped that same factor in the other direction.
  • 7.4525 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 8
Expected value of a discrete random variable — solve for outcome 2 (case 3) — Expected Values (8)

A student finds the expected value of a two-outcome gamble. Given outcome 1 (x1) = 1.5000; probability 1 (p1) = 0.3000; expected value (EX) = 9.4800, determine the outcome 2 (x2).

Given

  • outcome1(x1)=1.5000outcome 1 (x_{1}) = 1.5000
  • probability1(p1)=0.3000probability 1 (p_{1}) = 0.3000
  • expectedvalue(EX)=9.4800expected value (EX) = 9.4800

Find

outcome 2 (x2)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except x2 is given, so isolate x2 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for x2:

    x2=E[X]−x1p11−p1x_{2} = \dfrac{E[X] - x_1 p_1}{1-p_1}
  3. Step 3

    Listthegivens:outcome1(x1)=1.5000,probability1(p1)=0.3000,expectedvalue(EX)=9.4800List the givens: outcome 1 (x_{1}) = 1.5000, probability 1 (p_{1}) = 0.3000, expected value (EX) = 9.4800
  4. Step 4 — Substitute the given values:

    x2=E[X]−x1p11−p1x_{2} = \dfrac{E[X] - x_1 p_1}{1-p_1}
  5. Step 5 — Evaluate:

    x2=12.9000x_{2} = 12.9000
  6. Step 6 — Check: returning x2 = 12.9000 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
x2=12.9000x_{2} = 12.9000

Why the other options are there

  • 25.8000 — kept a factor of two that cancels in the correct rearrangement.
  • 6.4500 — dropped that same factor in the other direction.
  • 14.1900 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 9
Expected value of a discrete random variable — solve for probability 1 (case 3) — Expected Values (9)

The expected value of a production yield is estimated from probabilities. Given outcome 1 (x1) = 2.0000; outcome 2 (x2) = 20.0000; expected value (EX) = 12.0500, determine the probability 1 (p1).

Given

  • outcome1(x1)=2.0000outcome 1 (x_{1}) = 2.0000
  • outcome2(x2)=20.0000outcome 2 (x_{2}) = 20.0000
  • expectedvalue(EX)=12.0500expected value (EX) = 12.0500

Find

probability 1 (p1)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except p1 is given, so isolate p1 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for p1:

    p1=x2−E[X]x2−x1p_{1} = \dfrac{x_2 - E[X]}{x_2 - x_1}
  3. Step 3

    Listthegivens:outcome1(x1)=2.0000,outcome2(x2)=20.0000,expectedvalue(EX)=12.0500List the givens: outcome 1 (x_{1}) = 2.0000, outcome 2 (x_{2}) = 20.0000, expected value (EX) = 12.0500
  4. Step 4 — Substitute the given values:

    p1=x2−E[X]x2−x1p_{1} = \dfrac{x_2 - E[X]}{x_2 - x_1}
  5. Step 5 — Evaluate:

    p1=0.4417p_{1} = 0.4417
  6. Step 6 — Check: returning p1 = 0.4417 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
p1=0.4417p_{1} = 0.4417

Why the other options are there

  • 0.8833 — kept a factor of two that cancels in the correct rearrangement.
  • 0.2208 — dropped that same factor in the other direction.
  • 0.4858 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

Example 10
Expected value of a discrete random variable — solve for expected value (case 4) — Expected Values (10)

An engineer computes the expected value of a warranty payout distribution. Given outcome 1 (x1) = 7.5000; probability 1 (p1) = 0.3800; outcome 2 (x2) = 18.5000, determine the expected value (EX).

Given

  • outcome1(x1)=7.5000outcome 1 (x_{1}) = 7.5000
  • probability1(p1)=0.3800probability 1 (p_{1}) = 0.3800
  • outcome2(x2)=18.5000outcome 2 (x_{2}) = 18.5000

Find

expected value (EX)

Start with the thinking

  • The governing relation printed in this handbook section is Expected value of a discrete random variable.
  • Everything except EX is given, so isolate EX symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The expected value of a discrete random variable weights each outcome by its probability.

Step-by-step solution

  1. Step 1 — State the governing relation:

    E[X]=∑xipiE[X] = \sum x_i p_i
  2. Step 2 — Rearrange symbolically for EX:

    EX=x1p1+x2(1−p1)EX = x_1 p_1 + x_2(1-p_1)
  3. Step 3

    Listthegivens:outcome1(x1)=7.5000,probability1(p1)=0.3800,outcome2(x2)=18.5000List the givens: outcome 1 (x_{1}) = 7.5000, probability 1 (p_{1}) = 0.3800, outcome 2 (x_{2}) = 18.5000
  4. Step 4 — Substitute the given values:

    EX=x1p1+x2(1−p1)EX = x_1 p_1 + x_2(1-p_1)
  5. Step 5 — Evaluate:

    EX=14.3200EX = 14.3200
  6. Step 6 — Check: returning EX = 14.3200 to

    E[X]=∑xipiE[X] = \sum x_i p_i

    reproduces the given quantities, and both sides carry the same units.

Answer:
EX=14.3200EX = 14.3200

Why the other options are there

  • 28.6400 — kept a factor of two that cancels in the correct rearrangement.
  • 7.1600 — dropped that same factor in the other direction.
  • 15.7520 — rounded an intermediate value before the final step.

Reference: FE Handbook — Expected Values

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