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

Example Input: Problem: a lent rs . 5000 to b for 2 years and rs . 3000 to c for 4 years on simple interest at the same rate of interest and received rs . 1540 in all from both of them as interest . the rate of interest per annum is ?
Example Output: 7 %

Example Input: Problem: a cycle is bought for rs . 900 and sold for rs . 1080 , find the gain percent ?
Example Output: 20 %

Example Input: Problem: if the average ( arithmetic mean ) of a and b is 110 , and the average of b and c is 170 , what is the value of a − c ?
Example Output:
− 120