What is the probability of having 3 girls?

Answer 1

33% for every girl... (quick but wrong)

2. for a given solo pregnancy, we have the b-g option, or 1/2.

For two pregnancies, we have bb, bg, gb, gg.

for three pregnancies, we have bbb, bbg, bgb, bgg, ggg, gbg, ggb, gbb.

Of which some outcomes are equivalent in terms of numbers. (e.g. ggb, gbg, bgg)

However only one of the eight outcomes gives three girls, so the answer is the probablilty is 1/8.

An easier method, and necessary when the number are greater, is to say the probability is 1/2 in one pregnancy.

And since (for our purposes) the pregnancies are independent events, the cumulative probability is (1/2)3 which = 1/8 or 12.5% or 0.125.