P(exactly 3 tails on 4 coins) = P(3 tails on 3 coins) * P(1 tail on 1 coin) * (# of ways to arrange 3 t's and 1 h)
P(exactly 3 tails on 4 coins) = 1/8 * 1/2 * 4
P(exactly 3 tails on 4 coins) = 1/4
All combos: hhhh, hhht, hhth, hhtt, hthh, htht, htth, httt thhh, thht, thth, thtt, tthh, ttht, ttth, tttt
Want these: httt, thtt, ttht, ttth
Result: 4 / 16 = 1/4