35 percent of the books in Richard’'s collection are novels

Actually we don't need to know that to answer the question.

35 percent of the books in Richard’'s collection are novels and 60 percent of the books are paperbacks. If 20 percent of the paperbacks in Richard'’s collection are novels published before 1940, how many of the paperbacks in Richard’'s collection are novels published before 1940.

Say Richard has total of x books, then he has 0.6x paperbacks and 0.2*(0.6*x) paperback novels published before 1940. So, basically all we need to know in order to answer the question is the total number of book Richard has (x).

(1) Non-…ction books make up exactly half of Richard’s collection. Clearly insufficient.
(2) 100 of the books in Richard’s collection are non-…fiction. Clearly insufficient.

(1)+(2) Since 100 books (no matter which books) in Richard’s collection compose 50% (exactly half) of his collection, then the total number of books in the collection is 200, so Richard has 0.2*(0.6*x)=0.2*(0.6*200)=24 paperback novels published before 1940. Sufficient.

Answer: C.

Hope it's clear.