Posted by **Arun** on Thursday, March 1, 2012 at 12:30am.

The probability that the life time of randomly chosen device is below average, average,above average are respectively 0.2,0.5,0.3. The probability that of the 3 randomly chosen devices lifetimes of 2 are above average and 1 below average equals

a.0.054,b.0.018,c.0.036,d.0.012

- Probability and statistics -
**MathMate**, Thursday, March 1, 2012 at 8:55am
Think of the expansion of the polynomial

(A+B+C)^3 where A=below, B=averave, C=above, and A+B+C=1.

1 is below average = A (0.2)

2 are above average = C (0.3)

term=AC^2

The coefficient of AC^2 is 3, i.e. 3AC^2.

The probability is therefore

3*0.2*0.3^2=0.054

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