Question 1169686
find the probability of 0 boys and all girls.  That is what you don't want. What you want is 1-that value, since this is a dichotomy. 
There are 28 students and 28C3 ways to choose 3.
all girls would be 15C3/28C3
=455/3276
=0.1389
at least one boy chosen will have probability 1-0.1389=0.8611