To do the math step by step, first you need a common denominator. The easiest way to do this is multiply the bottom numbers together. --> 15•3 which is 45. This is your. Ew denominator. Now you need to make the top numbers match by multiplying the opposite number like this--> 15•1=15 so one fraction is now 15/45. Now the other one you do this--> 13•3= 39 so the other new fraction is 39/45. Now subtract 39/45 and 15/45 which is 24/45. Then simplify. 24 divided my 3 is 8 and 45 divided by 3 is 15. Now you can't simplify anymore so your answer is 8/15! :)