Mental maths
 

Try this one in your head (no pen/paper or calculator)

Take 1000 and add 40 to it. Now add another 1000. Now add 30.
Add another 1000. Then add 20. Now add another 1000. Finally add 10.
What is the total?
Did you get 5000?
The correct answer is actually 4100.
If you don't believe it, check it on a calculator.