I wish I had a brilliant mathematical answer, but it's been far too long since I have taken any math! I just did this via the brute, clumsy way of adding numbers. When I reached a total of 990, I knew the only way to reach exactly 1000 was with a 10.
Thanks! Here's a little follow-up: Inspired by the other questions about progressions, I just now worked this using the formula stated elsewhere, and it was very neat! If I practice it enough, I'll learn it well enough to use it (first) next time.
Another Math question on progressionA boy starts adding consecutive natural numbers starting from 1. He reaches a total of 575 when he realizes he has missed a number. What can be said about the missed number?