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

[EX Q]: Problem: if the cost price of 50 articles is equal to the selling price of 40 articles , then the gain or loss percent is ?
[EX A]: 25 %

[EX Q]: Problem: the compound interest on $ 30,000 at 7 % p . a is $ . 4347 . find out the period ( in years ) ?
[EX A]: 2

[EX Q]: Problem: a hollow iron pipe is 21 cm long and its external diameter is 8 cm . if the thickness of the pipe is 1 cm and iron weighs 8 g / cm 3 , then the weight of the pipe is :
[EX A]:
3.696 kg