It's heads or tails, so it's a binomial distribution. So we use the formula: ${n \choose r}{p^r}{(1-p)^{n-r}}$ where n is the number of trials (3) and r is the number of successes (tails). ${3 \choose 2}{0.5^2}{(1-0.5)^{3-2}}={3 \choose 2}{(0.25)}{(0.5)}={3 \choose 2}(0.125)$ ${n \choose r}={n! \over {r!(n-r)!}}={3! \over 2!1!}={{1*2*3} \over 1*2*1}=3$ So, $3*0.125=0.375=P(X=2)$