06/18/2020 (2011: Countdown Round, Problem 1)

Q: If Mark drives for 4 hours at 60 miles per hour and then drives another 6 hours at 40 miles per hour, what is his average speed, in miles per hour, for the trip?

A: 48

Mark traveled 4 * 60 = 240 miles in the first 4 hours and 6 * 40 = 240 miles in the next 6 hours. That means that Mark traveled 240 + 240 = 480 miles in 10 hours, so Mark's speed is 480/10 = 48 miles per hour.