Gratuity of $180 I would multiply the actual repair cost without including tax to determine the gratuity. So just multiply $1200 by 0.15 getting $180. As an suggestion, calculating 15% in your head is actually quite simple. What you do is calculate 10%, divide that in half, then add together. So for the $1200 example, you have 1200/10 = 120 half of 120 is 60 120 + 60 = 180 So you have a top of $180.