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

Ex Input:
Problem: the sum of the even numbers between 1 and n is 89 * 90 , where n is an odd number , then n = ?

Ex Output:
179

Ex Input:
Problem: a certain debt will be paid in 65 installments from january 1 to december 31 of a certain year . each of the first 20 payments is to be $ 410 ; each of the remaining payments is to be $ 65 more than each of the first 20 payments . what is the average ( arithmetic mean ) payment that will be made on the debt for the year ?

Ex Output:
455

Ex Input:
Problem: a and b together can complete a work in 12 days . a alone complete it in 20 days . if b does the work only for half a day daily , then in how many days a and b together will complete the work ?

Ex Output:
15 days