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Use binomial probability since there are only two possibilities: success and failure, where success represents getting a heads, and tails being a fail. Let $X$ = Success (i.e. heads) Therefore we are trying to find $P(X=2)$, which is $\binom42\cdot(0.5)^2\cdot(0.5)^2=0.375$. Hope this helped! The derivation of binomial probability: Getting two heads out of 4 can be portrayed is, disregarding order: HHTT (H=heads and T=tails) Multiplying their probabilities will yield $(0.5)^4$, but as for ordering, we get $4!/(2!\cdot2!)$ due to repetition, which is the same as $4C2$. So our answer is $\binom42\cdot(0.5)^4$ which is $0.375$ (责任编辑：) |