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

Example Input: Problem: by selling an article for $ 195 , a person gains $ 45 . what is the gain % ?
Example Output: 30 %

Example Input: Problem: for any integer n greater than 1 , n * denotes the product of all the integers from 1 to n , inclusive . how many prime numbers are there between 6 * + 2 and 6 * + 6 , inclusive ?
Example Output: none

Example Input: Problem: the sale price of a trolley bag including the sale tax is rs . 560 . the rate of sale tax is 12 % . if the shopkeeper has made a profit of 25 % , the cost price of the trolley bag is :
Example Output:
rs 400