Question

You earned $31,800 last year. How much money did you make each month?

Answer 1

You earned $2650 a month.

Answer 2

So last year you earned 31,800 dollars. 
So if you want to find out how much money you made each month, you have to divide the amount of money by the number of months.
There are 12 months in 1 year. 
So 31,800/12
=2,650
Each month you made $2,650.