If there are 25 cupcakes and someone spills 15 what do you divide

If you were looking for the percentage of the cupcakes that were spilled you would divide 15/25

Which would equal .6 or 6/10 simplified to 3/5 

If you were looking for the amount of cupcakes that you didn't spill you would subtract 25-15=10 and so therefore you would take 10/25 which would equal .4 or 2/5 

First, divide 25 by 15. That results in 0.6 or 6/10.
That means to get to 15, you have to divide by 0.6 or 6/10. :)