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

[EX Q]: Problem: if the average ( arithmetic mean ) of a and b is 45 and the average of b and c is 80 , what is the value of c − a ?
[EX A]: 60

[EX Q]: Problem: at what rate percent on simple interest will rs . 750 amount to rs . 900 in 8 years ?
[EX A]: 2.5 %

[EX Q]: Problem: what is the rate percent when the simple interest on rs . 810 amount to rs . 155 in 4 years ?
[EX A]:
4.78