In February 2002, L. N. Trefethen published a list of 10 short problems. They were all stated in a short paragraph, and their answer was a single nmber. It was required to compute all 10 numbers to 10 digits accuracy.
My first thought was that they were all very hard. I knew I could do number 8, at least in principle. I started thinking about some of them, and slowly realized that I knew how to solve most of the problems, then all of them (or at least I could find software for solving them).
The solution I submitted has 99 correct digits (the 10th digit in problem 2 is wrong), and was awarded Second Prize (together with 4 others).