In areas and circles, you have to learn formula for area and perimeter and you have to find out what are the values you needed for the problem to solve and what to find out.

Compound Interest - again here there is formula for simple interest that you have to by heart and once you find simple interest you can calculate the compound interest.

Average - Always you divide by the total number of values given.

When we talk about mathematics the main thing is logic. you have find out the logic that is hidden in a problem only then it will be easy for you to do the problems and formulas, no other way to go you have by heart wherever it is needed.