8. Instead of regular air, scuba divers on deep dives sometimes breathe hellox, mixture of oxygen and helium. An engineer wants to test the process of filling and emptying a large storage tank for heliox. The tank has a volume of 400,000 cm. The rate at which the tank can be filled is 5,000 cm per hour faster than the rate it can be emptied. Suppose r is the rate at which this tank is filled, in thousands of cm per hour. Write an equation for t, the total number of hours it takes to empty and fill the tank, in terms of r.