I see a pattern here! Let me show you - multiply 15 by the week number (1, 2, 3, 4, ...), and you have y.
For example, 15 x 3 (for week 3) is 45, as shown by the y value.
So all we must do is multiply 15 by 5, and voila! We'll have our answer. Which, by the way, is 75.
I hope this answers your question!
-Toremi
