If you run for 0.4 hours at 7 mph, how fast should you walk during the next 0.8 hours to have the average speed of 5 mph?

4 mph

Step-by-step explanation:

Average speed is total distance over total time.

If you run for 0.4 hours at 7 mph, the distance you run is:

d = rt

d = (7 mi/hr) (0.4 hr)

d = 2.8 mi

If you walk for 0.8 hours at x mph, the distance you run is:

d = rt

d = (x mi/hr) (0.8 hr)

d = 0.8x mi

So your average speed is:

5 mi/hr = (2.8 mi + 0.8x mi) / (0.4 hr + 0.8 hr)

5 mi/hr = (2.8 mi + 0.8x mi) / 1.2 hr

6 mi = 2.8 mi + 0.8x mi

3.2 mi = 0.8x mi

x = 4

You should walk at 4 mph.