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Random Variables, Expectation & Variance · Question 66 of 68

Question 66

MediumOriginal question

A seller earns ₹10 on a day with probability 0.3 and nothing with probability 0.7. What is the variance of the day's earnings, in rupees squared?

Options for question 66

Show the answer and explanation
CorrectNot quiteAnswer C

Correct answer: C

Explanation

  1. E=10×0.3=3E = 10 \times 0.3 = 3 and E(X2)=100×0.3=30E(X^{2}) = 100 \times 0.3 = 30.

  2. Var=30−9=21\text{Var} = 30 - 9 = 21.

Worth noting

30 is E(X2)E(X^{2}) and 9 is the square of the mean.

Concept
The variance from a table
Typical time
90 s
Your time
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The rule this testsVariance and standard deviation
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