Many choices: Independent events
Because we need 6 books, we want to choose 2 books from each the followings:
-- 5 books of Mathematics,
-- 4 books of Physics, and
-- 2 books of Biology.
How many possibilities do we have?
We have:
C(2,5) = 10 possibilities to choose 2 books among the 5 books
of Mathematics.
C(2,4) = 6 possibilities to choose 2 books among the 4 books
of Physics.
C(2,2) = 1 possibility to choose 2 books among the 2 books
of Biology.
The total of possibilities is: 10 x 6 x 1 = 60 possibilities
to choose 3 books among 11 books.