If two coins are tossed simultaneously,
S = {HH, HT, TH, TT} n(S) = 4
(i) Condition for event A : at least one head.
A = {HH, HT, TH} n(A) = 3
(ii) Condition for event B : only one head.
B = { HT, TH} n(B) = 2
(iii) Condition for event C : at most one tail.
C = {HH, HT, TH} n(C) = 3
(iv) Condition for event D : No head.
D = {TT} n(D) = 1