If the probability of rain on any given day in City X is 50%, what is probability that it rains on exactly 3 days in a 4 day period?  Choose the correct choice from the following choices: A) 1/2 B) 1/4 C) 1/8 D) 1/16 E) 3/16
One possible way is rain-rain-rain-no rain.   The probability of this is 1/2*1/2*1/2*1/2 = 1/16   The number of possible ways is 4C3 = 4, so we multiply this by 4.   P(rain on exactly 3 days) = 4*1/16 = 4/16 = 1/4   The answer is 1/4.
<result>B</result>
If the probability of rain on any given day in City X is 50%, what is probability that it rains on exactly 3 days in a 4 day period?