[求助]prep上的一道math

angelavivi

if a coin has an equal probability of landing heads up or tails up each time it is flipped, what is the probability that the coin with land heads up exactly twice in 3 consecutive flips?

answer:0.375

why?

Thank you for help.

meimei5656

 C(2,3)*1/2^3=.375

angelavivi

哦 明白了! 我怎么这么晕,做题时候肯定大脑短路了.