Tomorrow a delivery truck will run a route 20% longer than today's, and it will run 10% faster than it did today. The time the trip takes will be greater than today's by what percent?

Time = distance ÷ speed, so the time is multiplied by 1.20/1.10 = 1.0909 — a rise of 9.09%. (Subtracting the two rates to get 10% would be right only if time were distance MINUS speed; adding them gives 30%, which is wrong twice over. 8.33% is the factor upside down, 1.10/1.20 = 0.9167, and describes a shortening that does not happen.)