Tony exercises every morning by walking 3 miles carrying a 25-pound backpack, then he runs another 10 miles without the backpack. If he walks at a speed of 3 miles per hour and runs at a speed of 5 miles per hour, how many hours each week does he spend exercising? Let's be accurate as possible.
Tony walks 3 miles at 3 miles per hour for 3 / 3 = 1 hour. Tony also runs 10 miles at 5 miles per hour for 10 / 5 = 2 hours. Thus, each day he exercises for 1 + 2 = 3 hours. Over the course of one week, he exercises for 3 * 7 = 21 hours.
The answer: 21.