<human>:  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?
<bot>: How many hours does Tony walk? ** Tony walks 3 miles at 3 miles per hour for 3/3=1 hour.
How many hours does Tony run? ** Tony also runs 10 miles at 5 miles per hour for 10/5=<<10/5=2>>2 hours.
How many hours does Tony exercise each day? ** Thus, each day he exercises for 1+2=<<3=3>>3 hours.
How many hours does Tony exercise each week? ** Over the course of one week, he exercises for 3*7=<<3*7=21>>21 hours.
#### 21