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If the probability of rain on any given day in Chicago

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If the probability of rain on any given day in Chicago [#permalink]

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20 Dec 2006, 03:27
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If the probability of rain on any given day in Chicago during the summer is 50%, independent of what happens on any other day, what is the probability of having exactly 3 rainy days from July 4 through July 8, inclusive?

(A) 1/32

(B) 2/25

(C) 5/16

(D) 8/25

(E) 3/4
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20 Dec 2006, 04:35
C

Probabilty of rain on a day = 50 % = 1/2 ( say R)
probability of no rain on a day = 1 - (1/2) = 1/2 ( say NR)

So probability of rain on 3 days out of 5 days
= 5C3 ( R)^3 ( NR)^ (5-2)
= 5/16
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20 Dec 2006, 05:14
Yes C...

Total number of outcomes: 2*2*2*2*2=32
Number of favourable outcomes: 5C3=10

Probability=10/32=5/16
20 Dec 2006, 05:14
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