Have you tried changing both fractions to a common denominator yet ?
You can't add or subtract fractions until they have the same denominator.
Here's a hint:
The smallest common denominator for 15ths and 12ths is 60ths.
Once both of your fractions have the same denominator, all you have to do is subtract the second numerator from the first one, and put that result over the same denominator, and the answer is right there in front of you.
Here's a deal: I'll give you the answer, but DON't hand it in on a homework sheet and say that you did it. I want you to follow the instructions that I explained up above, and use my answer only to see when you did it correctly. THAT way, if you don't watch out, you might actually LEARN something !
Answer to check your work against: 7 / 20 or 21 / 60 .
