Number of possible outcomes:
(H,T),(T,H),(H,H),(T,T) = 4
Therefore four outcomes are possible.
Number of head once in every outcomes:
Here (H,H) is included because head comes atleast once in this outcome.
So the number of head once in every outcomes = 3
Probability = Total outcomes / possible outcomes
= 4 / 3
Therefore number of head appearing once is of probability 4/3