First let's find 10% of the bill. 38.7 divided by 10 = $3.87. Now we can add this to the bill until we get the answer or close to it. 38.70 + 3.87 = 42.57 + 3.87 = 46.44. That is the total cost. How many times did we add it? Twice, so we do 10*2=20. The tip was 20% of the bill
