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

Example Input: Problem: calculate the sum of first 60 natural numbers .
Example Output: 1830

Example Input: Problem: excluding stoppages , the speed of the bus is 54 kmph and including stoppages , it is 36 kmph . for how many minutes does the bus stop per hr
Example Output: 20 mi

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 .