The fact that the first chosen student was a girl is already decided, so there is no need to find the probability of that happening. There are a total of 13+10, or 23 students. A girl was already selected, so that leaves 22 students. The chance that a boy is chosen is 10/22, or 5/11.
Answer: sin pi/4 sin pi/6 = 1/2(cos pi/12 - cos 5pi/12)
correct on apex
Since it's 3 people splitting 10 oz, we get:

Which can then be simplified to a mixed fraction:

So, yes, you got it right! Good job!