You will be given a definition of a task first, then some input of the task.
The task is to generate the correct answer for a given math problem.

Problem: the compound interest earned by sunil on a certain amount at the end of two years at the rate of 10 % p . a . was rs . 420 . find the total amount that sunil got back at the end of two years in the form of principal plus interest earned .
Output:
rs . 2420