Q: The av. birth rate in a hospital is 30 births per day. In a given day 25 boys are born. How many girls do you expect to be born on that day?

(assume the prob. of being a boy is 1/2 and is independent of the total number of births)

I'm bit confused, am I supposed to use binomial/poisson distributions? how am I supposed to find a expected value instead of a probability? Or is it really simple?

thanks for the help.