DAY 223

You are going to run 12 miles and hope to do it at an average speed of 6 miles per hour. You are running uphill and when you arrive at the halfway point (the top of the hill) you realize you have averaged only 3 miles per hour. How fast must you run downhill to make the entire round trip's average speed 6 miles per hour.

Answer:

Sorry, unless you have a time machine, it's not going to happen. To run 12 miles at 6 miles per hour would take two hours. When you get to the top of the hill, you have run 6 miles. At an average speed of 3 miles per hour, this has taken you 2 hours. You have used up all your time and are only half-way through the run.