This riddle must be done IN YOUR HEAD and NOT using paper and pen or a calculator. Try it!
Take 1000 and add 40 to it. Now add another 1000 to it.
Now add 30. And another 1000.
Now add 20. Now add another 1000. Now add 10.
What is the total?

Did you get 5000? Way to go. Most of us did, but that's wrong. The answer is 4100. Believe it: use a calculator.