You are absolutely right, excellent logic. Basically, k = number you have to multiply the numbers from x axis(earned) to get numbers from y axis(paid), so you simply pick one point which you know the coordinated and then divide the values of y by the value of x, for example : k = y/x = 25/10 = 50/20 = 75/30 = 87,50/35 = 2,5 . It doesn't matter which point you pick, it's always gonna be the same in this case. Hope this helps.