Mary's sheep can run 12 feet per second, and her sheepdog can run 20 feet per second. A sheep standing 160 feet away from the sheep dog bolts, and the dog runs after it. How many seconds does it take the dog to catch the sheep? Think carefully first, then make a decision:
First find how much faster the dog is than the sheep: 20 feet / second - 12 feet / second = 8 feet / second. Then divide the distance the dog needs to run by the speed difference: 160 feet / 8 feet / second = 20 seconds. So, the answer is 20.