Easier when your number after the decimal (0.xxx) is short. Then all you need to do is multiply that number with the number after the division.
Ex: 32 % 12 = 8
You do 32/12=2.666666667
Then you throw the 2
away, and focus on the 0.666666667
0.666666667*12=8
<-- That's your answer.
(again, only easy when the number after the decimal is short)