LetsassumeX~Binomial(n=no of trials, p= probability of success)

The binomial theorem states

In your case lets call X the probability of having a girl with and

a) find

b) exactly 19 boys = exactly 1 girl so find

c) "at least 1" means 1 or more so find

Remember

d) "at most 1" means 1 or less so this means 2 or more girls now find