Question: what is 20 of a tip?
Answer: Since 20% is 2 times 10%, 20% of the bill is 2.65 + 2.65 = 5.30. Now you know the tip you could leave ranging from poor to good, $2.65 to $5.30. To find 15%, halfway in between, we know 15% is 10% plus 1/2 of 10% (5%).