The task is to generate the correct answer for a given math problem.

Example Input: Problem: robert ' s salary was decreased by 20 % and subsequently increased by 20 % . how much percentage does he lose ?
Example Output: 4 %

Example Input: Problem: carl is facing very difficult financial times and can only pay the interest on a $ 10,000 loan he has taken . the bank charges him a quarterly compound rate of 8 % . what is the approximate interest he pays annually ?
Example Output: 3500

Example Input: Problem: keats library purchases a number of new books , all in the category of biography , and the library does not acquire any other books . with the addition of the new biographies , the biography collection of the library amounts to 30 % of the total number of books in the library . if prior to the purchase , only 20 % of the books in keats library were biographies , by what percent has the number of biographies in the library increased ?
Example Output:
71 %