We can quickly calculate a ten percent tip by moving the decimal place one to the left. So a 10% tip on $85.00 would be $8.50. We can then take half of that tip to calculate the remaining 5%. $8.50 / 2 = $4.25. The sum of these two results is equal to a 15% tip. $8.50 + $4.25 = $12.75. The correct answer is D.