Throw those darts #2

Harbo designed the second dart game. Its target board also had three circles with the numbers 100, 300, and 900 in them.

The game was simple. A player threw three darts. The score was figured out by adding up the numbers in the circles where the darts hit. To plan for the prizes, Harbo needed to know all of the different scores that were possible when all three darts hit the circles. He started doing some arithmetic on the sand floor.