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

Ex Input:
Problem: a can do a work in 15 days and b in 20 days . if they work on it together for 7 days , then the fraction of the work that is left is

Ex Output:
11 / 6

Ex Input:
Problem: how many liters of a 40 % iodine solution need to be mixed with 35 liters of a 20 % iodine solution to create a 34 % iodine solution ?

Ex Output:
13

Ex Input:
Problem: if 3 people can do 3 times of a particular work in 3 days , then how many days would it take 9 people to do 9 times of that particular work ?

Ex Output:
3