<p>the question is if a coin is flipped 4 times, what is the chance of getting heads twice? they say the answer is 3/8. how is it not 1/2?</p>
<p>the chance would be 1/2 if the question was asking the chance of getting heads on one flip. instead, you have to do it like this:</p>
<p>4 nCr 2 = 6 = number of combinations with two heads
(1/2) - chance of getting heads on one flip, (1/2)^2 - chance of getting heads on two flips, (1/2)^2 chance of getting tails on two flips (remember, the other two would be tails), (1/2^2*1/2^2)=1/2 ^4.
6((1/2)^4) = 3/8</p>