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

Example Input: Problem: by selling an article for $ 225 a person gains $ 75 . what is the gain % ?
Example Output: 50 %

Example Input: Problem: in an election between two candidates , one got 60 % of the total valid votes , 30 % of the votes were invalid . if the total number of votes was 9000 , the number of valid votes that the other candidate got , was :
Example Output: 2520

Example Input: Problem: how much time will a train of length 380 m moving at a speed of 72 kmph take to cross another train of length 540 m , moving at 36 kmph in the same direction ?
Example Output:
92