Solve this using only your mind, without a calculator or paper.

by

Add 40 first, then 1000, then 30 next, then 1000, then 20 next, then 1000, and lastly, 10

The sum is:

4090 is the sum of 1000 + 40 + 1000 + 30 + 1000 + 20 + 1000 + 10