Question 1182308
for 2 each it would be 5C2*4C2/9C4, the 9C4 is choosing 4 books from 9. It is also the "sum" of the two top terms.
=10*6/126 or 60/126=10/21
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for at least 1 on each topic. Look at the possibility of none for each topic. That is 4C0*5C4/9C4=5/126
and 5C0*4C!/9C4=4/126.  We want everything but these two possibilities, which is 117/126=13/14.