A couple plans to have three children. What is the probability that
prob(no boys) = (1/2)^3 = 1/8
prob(1 b, 2g) = 3(1/2)^3 = 3/8
prob(2b, 1g) = 3(1/2)^3 = 3/8
prob( 3 boys) = 1/2 ^3 = 1/8
those are all the cases, note 1/8+3/8+3/8+1/8 = 1
prob(1 b, 2g) = 3(1/2)^3 = 3/8
prob(2b, 1g) = 3(1/2)^3 = 3/8
prob( 3 boys) = 1/2 ^3 = 1/8
those are all the cases, note 1/8+3/8+3/8+1/8 = 1