Need help with Calculus 1.?

Hey guys! Need help from some math buffs. I'm tutoring a friend in calculus while her regular tutor is out of town and I need some help with a word problem her professor assigned. It's been 5 years since I've taken calculus 1, but I think the biggest problem I'm having is her professor's lack of clarity when writing his word problems. Any help is greatly appreciated. And keep in mind that I'll need to explain to her how to do this, as well. We're working with derivatives. "Two runners are running on circular tracks each of which has a circumference of 400m. The tracks are 100m apart and the runners start opposite each other and move at the same constant rate of 400m/min. How fast are the runners separating when each has run 10m?" So obviously I need their rate of separation given they're initially 100m apart. Thanks!