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Probability problem

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by maxim730 » Sun Dec 31, 2006 4:02 pm
Q: The probability to rain on a given day is 25%. What is the probability that it does not rain during a 2-day period?

A: 8/125, 2/25, 5/16, 9/16 or 3/4

OA is D, 9/16

I can't believe I got this wrong. Here was my method, what did i do wrong?

1 - (prob to rain) = prob NOT to rain

prob. to rain = 1/4*1/4 = 1/16 during 2 day period

1 - (1/16) = 15/16.

How is this incorrect? What step am i missing? :( Thanks!!

EDIT: Ok I guess They just multiple 3/4 * 3/4 = 9/16 to get the answer.

But why wouldn't it work the other way: 1 - (prob. to occur) = (prob. not to occur)??
Happy New Year!!
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Senior | Next Rank: 100 Posts
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by thankont » Sun Dec 31, 2006 11:31 pm
you forgot the fact that it could rain on one day and not on the other and the other way around so you actually have 4 cases
RR. RS, SR, SS so for both days to be sunny we either have 3/4*3/4 =9/16 or 1 - (1/16+3/16+3/16) = 1-7/16 = 9/16
Use the binomial distribution to attack these kinds of problems...
thanx
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