Each child born to a particular set of parents has probability 0.25 of having blood type o. if these parents have 5 children, what is the probability that exactly 2 of them have type o blood?

Use binomial distribution with p=0.25, n=5, x=2

P (X=x) = C (n, x) p^x (1-p) ^ (n-x)

P (X=2) = C (5,2) 0.25^2 0.75^ (5-2)

=10*0.0625*0.421875

=0.26367

So the probability of 2 of 5 children having type-O blood from these parents is 0.26367.