instruction:
The task is to generate the correct answer for a given math problem.
question:
Problem: by selling an article for $ 195 , a person gains $ 45 . what is the gain % ?
answer:
30 %

question:
Problem: a swimmer can swim in still water at 4 km / h . if the speed of the water current is 2 km / h , how many hours will the swimmer take to swim against the current for 5 km ?
answer:
2.5

question:
Problem: if a 2 - b 2 = 10 and a * b = 5 , find a 4 + b 4 .
answer:
150