This is a permutation (without replacement) so the probability is the probability of the first event times the probability of the second event. The equation would be ((x)/16)*((x-1)/(16-1))=1/12 don't ask me how I got this I'm not even sure myself. Algebra will give you an answer of x=5 or x=-4 and there can't be -4 girls in the class so there must be 5 girls.
