Older Younger ------------------ A Girl Girl B Girl Boy C Boy Girl D Boy BoyEach of these combinations is equally likely (i.e. there is a one in four chance that any two-child family will have one of the above combinations).
For Mrs. Jones, the possibilities are narrowed down to A, B or C and of these, only A means that both are both boys. Therefore, the chance that both her children are boys is one in three.
For Mrs. Brown, A and C are the only possibilities. There is thus a one in two chance that both her children are boys.