Ask someone to write a number, say a 5-digit number. (Can be a 2-digit.....3-digit.....4-digit....etc...…).

Suppose the number written is 78513.

Now, tell the person that you are each going to write down 2 more 5 digit numbers and add them, but before you do, you are going to write down the answer on a piece of paper and keep it folded.

- thus 78513 become 278511 for the answer.
Now ask the person to write two more 5-digits number down, either at once or in succession with you. Either way, he must go before you.

Suppose he writes 27569.

So you now have...

78513

27569

27569

Now you write a five-digit number below it in such a way that each digit is 9 minus digit above. You now have...

78513

27569

72430

27569

72430

Then if he adds 45732, you add below it 54267. Note that you decided the number by subtracting each of his digits from 9.

78513

27569

72430

45732

54267

27569

72430

45732

54267

Now, ask him to add all the number and he gets 278511.

Show him the number which you had written as the answer to this addition earlier in the folded paper and … you're a hero!

Another example:

Suppose the number written is 746. You write 2744 on a piece of paper for the answer and fold it up.

Original number 746

Their first number 325

Your first number 674 (999-325)

Their second number 841

Your second number

__158__(999-841)
Total is

**2744**
They unfold the paper and you're a hero!

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