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

Ex Input:
Problem: if a - b = 5 and a 2 + b 2 = 31 , find the value of ab .

Ex Output:
3


Ex Input:
Problem: a car salesman earns a base salary of $ 1000 per month plus a commission of $ 200 for each car he sells . if the car salesman earned $ 2000 in march , how many cars does he need to sell in april in order to double his march earnings ?

Ex Output:
15


Ex Input:
Problem: a women purchased 3 towels @ rs . 100 each , 5 towels @ rs . 150 each and two towels at a certain rate which is now slipped off from his memory . but she remembers that the average price of the towels was rs . 165 . find the unknown rate of two towels ?

Ex Output:

e ) 60