The problem of the 100 prisoners:


Choosing randomly 50 drawers,
the changes to find your own number is 50 %.
In this way, the probability, that all prisoners are pardoned, is
(0,5)^{100} ≈ 0,0000000000000000000000000000008.
But there is a strategy, that all prisoners are pardoned,
with probability ≈ 31 %.