P (youngest is a girl) = 1/2, P (at least one is a girl) = 1/3 P (youngest is a girl) = 1/3, P (at least one is a girl) = 1/2P (youngest is a girl) = 1/3, P (at least one is a girl) = 1/4P (youngest is a girl) = 1/2, P (at least one is a girl) = 2/3