Q: I drive at an average speed of 30 miles per hour to the railroad station each morning and just catch my train. On a particular morning there was a lot of traffic and at the halfway point I found I had averaged only 15 miles per hour. How fast must I drive for the rest of the way to catch my train?

A: It’s impossible. 
Let’s say that you need to go 30 miles which would take an hour.
If you were at the halfway point, 15 miles, at 15 miles per hour, you have already been traveling for an hour  to go that 15 miles — and you are only halfway there!
Sorry…you will miss your train.