
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: girlgirl, girlboy, boygirl, and
boyboy. Given that I have one sister, the boyboy
combination is impossible. So in this case, there
are three possible combinations, and only one with
both girls. 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 girlboy
option. This leaves only two choices. STRATEGY: Draw a picture or diagram. 
