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

Example Input: Problem: if 4 xz + yw = 10 and xw + yz = 20 , what is the value of the expression ( 2 x + y ) ( 2 z + w ) ?
Example Output: 50 .

Example Input: Problem: the average ( arithmetic mean ) of 8 numbers is 43.1 . if the sum of half of these numbers is 158.4 , what is the average of the other half ?
Example Output: 46.6

Example Input: Problem: the average of 50 numbers id 20 . if two numbers , namely 45 and 55 are discarded , the average of the remaining numbers is :
Example Output:
18.7