Okay, so let's think it through a little bit. You need to find out how much money Byron paid his friends. You know Byron has four friends and that he paid each of his friends the same amount. So, let's declare a variable. We'll let the amount of money Byron paid to each one of his friends be represented by the letter d. At the start, there were three friends, so how would you represent the total amount of money Byron paid? The problem tells you that when the fourth friend showed up, the amount each friend was reduced by 15 dollars. So how would you create a new expression to reflect how much money Byron paid? Then ask yourself, did the total amount of money Byron paid change when the fourth friend came? With that answer in mind, how can you relate your two expressions from before?
Hopefully that's enough to get you thinking in the right direction. If you're still stuck, please include all of your steps and work (even if you know it's wrong) when you reply. Good luck.