||The probability that both are girls is 1/3. (The
odds that both are girls is 1 to 2- one way it can
be true against two ways it can not.) There are
four possibilities when considering the gender of
two siblings: girl-girl, girl-boy, boy-girl, and
boy-boy. Given that I have one sister, the boy-boy
combination is impossible. So in this case, there
are three possible combinations, and only one with
In the second case, the probability of having
two brothers is 1/2. (The odds are "even" - 1
to 1.) The same four possibilities for gender
must be considered. Girl- girl is not possible
from the given information. The statement "the
older one is a boy" also eliminates the girl-boy
option. This leaves only two choices.
STRATEGY: Draw a picture or diagram.