I'm having trouble solving this question...I'll try to be more specific. The problem could be split into 3 "cases" (4 bio, 3 bio+1 novel, 2 bio+2 novels) I used combination rule for 4 bio case and got 1.
The 3 bio+1 novel is the tricky part I can't seem to solve and need help on.
Also, is there any shorter/faster approach of solving this problem?
Hint: If only 2 biographies are chosen then there are $\binom{4}{2}$ ways to pick those biographies and $\binom{6}{2}$ ways to pick other books. So there are $\binom{4}{2}\cdot\binom{6}{2}=6\cdot 15=90$ ways to pick the books.
Now repeat this analysis when the reader picks exactly 3 biographies and 4 biographies and add all your answers together.