This must be done in your head only Do NOT use paper and pencil or a calculator. Try it. Take 1000 and add 40 to it. Now add another 1000 . Now add 30. Add another 1000 . Now add 20 . Now add another 1000 Now add 10. What is the total? Did you get 5000 ? The correct answer is actually 4100. If you don't believe it, check it with a calculator!
You can get out of the confusion when you first add all the 1000s and then add the two digit numbers. That will save you from the bluff