Instructions: In this task, you need to provide the correct option for a given problem from the provided options.
Input: Problem: in a class of 40 students , 10 did not opt for math . 15 did not opt for science and 2 did not opt for either . how many students of the class opted for both math and science ? 
Options: a ) 13 , b ) 15 , c ) 7 , d ) 17 , e ) 18
Output:
c