Three families each have three sons and no daughters. Assuming boy and girl babies are equally likely, what is the probability of this event?

I don't think it matters how many families are involved.

You're talking nine live births, and the probability of a

male birth each time is 50%.

So the probability of nine males in nine births is

(1/2) ^9 = 1/512 = about 0.195 percent (rounded)

Three families with three children all boys

parents-2

children-3

total-5 times 3 = 15

then boys-3 times three=9

Answer: 9/12

Percentage:75%