The probability of a radioactive atom decaying within its half life is 50%.
Probability of not decaying in 1 hour = 0.5
Probability of not decaying in another 1 hour = 0.5
Therefore, the total probability of not decaying in 2 hrs =0.5×0.5
=0.25=25%
So, probability of decaying in two hours = (100−25)%=75%