What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?
I drive at an average speed of 30 miles per hour to the railroad station each morning and catch my train. On a particular morning, there was a lot of traffic and at the halfway point, I had averaged only 15 miles per hour. How fast must I drive for the rest of the way to catch my train?