CA Foundation · Quantitative Aptitude · Probability
A random variable X takes values 1, 2 and 3 with probabilities 0.2, 0.5 and 0.3. What is Var(X)?
Variance is E(X²) minus the square of E(X). Here E(X) is 2.1 and E(X²) is 4.9, so the variance is 4.9 minus 4.41, which equals 0.49.
- A0.49Correct
- B0.61
- C0.70
- D0.81
Explanation
E(X) = 0.2+1.0+0.9 = 2.1. E(X²) = 0.2+2.0+2.7 = 4.9. Var = 4.9 - 2.1² = 4.9 - 4.41 = 0.49. Option 0.70 is wrong because it uses E(X²) minus E(X) incorrectly.
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One question tells you little. A timed set on Probability shows your real accuracy, how long you take and where you lose marks.
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