Consider the question. Mark has two pets, a hare that runs 10 feet/second and a turtle that crawls 1 foot/second. If they're going to run a 20 foot-race, how much of a head start (in seconds) does the turtle need to finish in a tie?
 If the answer is '18'; explain the reasoning:
We can find how long the hare spends running by dividing the distance of the race by the hare's speed: 20 feet / 10 feet / second = 2 seconds. We can do the same for the turtle: 20 feet / 1 foot / second = 20 seconds. Finally, subtract the hare's run time from the turtle's run time to find the length of the head start the turtle needs: 20 seconds - 2 seconds = 18 seconds.