Question: A large and a small heating unit are being used to heat the water of a pool. The larger unit, working alone, requires 8 h to heat the pool. After both units have been operating for 2 h, the larger unit is turned off. The smaller unit requires 9 h more to heat the pool. How long would it take the small unit, working alone, to heat the pool? (For full credit on word problems you must solve an appropriate equation.)