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Quote:
23. The best answer is C.
We have 7!/(4!*3!) = 35 different possibilities for 4 days of rain out of 7 consecutive days (choosing 4 out of seven). Every one of these 35 possibilities has the following probability: every day has the chance of ½ to rain so we have 4 days of ½ that it will rain and 3 days of ½ that it will not rain. We have ½ to the power of 7 = 1/128 as the probability of every single event. The total is 35 x 1/128 = 35/128.
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35 possibilities for picking any 4 days out of 7 days is alright. But the problem says 4
consecutive days. That only leaves 4 ways (consecutive days starting 1st day, 2nd day, 3rd and 4th of the seven day period). So how 35 possibilities?