There are as many books in the two libraries. If A gives B 1, and B is twice as many as A, how many books do you want?
Suppose A has xN books, and B has yN books (N is hundreds or thousands or tens or millions) 1, and X = Y 2,2 * (X-1) = Y+1. Solution: Originally A had 3N books and B had 3N books. Now a has (3N- 1.