Sample space obtained by throwing a coin four time
S = {HHHH, HHHT, HHTH, HHTT, HTHH, HTHT, HTTH, HTTP, THHH, THHT, THTH, THTT, TTHH, TTHT, TTTH, TTTT}
n(S) = 16
Let appearance of tail at least 3 times is event A, then
A = {HHHH, HHHT, HHTH, HTHT, HTHH}
n(A) = 5
Thus, required probability
P(A) = n(A)/n(S) = 5/16
Thus, probability to get tail at least 3 times is 5/16