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

[Q]: Problem: if a - b = 5 and a 2 + b 2 = 31 , find the value of ab .
[A]: 3


[Q]: Problem: a and b can do a piece of work in 7 days . with the help of c they finish the work in 5 days . c alone can do that piece of work in ?
[A]: 30 days


[Q]: Problem: a glass was filled with 10 ounces of water , and 0.03 ounce of the water evaporated each day during a 20 - day period . what percent of the original amount of water evaporated during this period ?
[A]:
6 %