We can figure out how long Michael's trip took by realizing that 30 mph is 3 times 10 mph. Because of this, the trip home will take 1/3 the amount of time it took to go up the hill. This makes the equation 24 + 8 = 32 minutes for the whole trip.
Now we need to figure out how long the trip was. We can do this by multiplying 10 minutes by 24/60, which equals 4. This means that Michael went 4 miles there, and 4 miles back. So Michael went a total of 8 miles.
Now we know that Michael went 8 miles in 32 minutes. Now, we multiply 8 by 60/32. This gives us 15, so Michael went 15 miles per hour on average in the trip.
