Ask your friends to try this simple sum. Read it out just as it appears below. Don’t let your friends use a pencil and paper or a calculator-they must figure it out in their heads.

Take 1000 and add 40 to it.

Now add another 1000.

Now add 30.

Now add another 1000.

Now add 20.

Add another 1000.

Finally add 10.

What’s the total?

Your friends will probably say the answer is 5000. Congratulations, you are a maths magician, because this is the wrong answer. The right answer is 4100.

If your friends don’t believe you, make them do the sum again using a calculator as you rad the instructions aloud.