Chapter 11, Problem 5TYU

Assume that the ratio of females to males is 1:1. A couple already has two daughters and no sons. If they plan to have a total of six children, what is the probability that they will have four more girls? (a) ¼ (b) ⅛ (c) $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$16$}\right.$ (d) $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$32$}\right.$ (e) $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$64$}\right.$