i found this question over the internet from an aptitude practice exam.

cant solve this problem..

below are the Question and Answer.

Q > if 2/5 of 3x=120, what is 1/5 of x?

A > First we find what 2/5 of 1x is by dividing 120 by 3=40.

if 2/5 of x=40, then 1/5 of x must be 40/2=20.

may i know what topic should i study so i can understand this problem?

Thanks