joe drives 420 miles at 60 miles per hour , and then he drives the next 120 miles at 40 miles per hour . what is his average speed for the entire trip in miles per hour ?
"t 1 = 420 / 60 = 7 hours t 2 = 120 / 40 = 3 hours t = t 1 + t 2 = 10 hours avg speed = total distance / t = 540 / 10 = 54 mph = b"
divide(add(420, 120), add(divide(420, 60), divide(120, 40)))