A coin is tossed twice so the sample space will be
{ (H,T) (H,H) (T,T) (T,H) } = 4
now the head come once in two tosses will be { (H,T) (H,H) } = 2
now the probability P = 2/4 = 0.5

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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
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