How to find $10 + 15 + 20 + 25 + \dots + 1500$ During a test I was given the following question:
What is $$10+15+20+25+...+1490+1495+1500=?$$
After process of elimination and guessing, $ came up with an answer.  Can anyone tell me a simple way to calculate this problem without having to try to actually add the numbers?

Hint 3 (Gauss): $10 + 1500 = 15 + 1495 = 20 + 1490 = \cdots = 1500 + 10$.
