Here it is!
Go and hide in a cupboard, or cover your eyes in some way so that you can't see what your
friend is writing.
Ask a friend to secretly write down ANY number (at least four digits long). e.g. 78341
Ask the friend to add up the digits... e.g. 7+8+3+4+1 = 23
... and then subtract the answer from the first number. e.g. 78341 - 23 = 78318
Your friend then crosses out ONE digit from the answer. (It can be any digit except a zero) e.g.
7x318
Your friend then reads out what digits are left .e.g. 7-3-1-8
Even though you haven't seen any numbers, you can say what the missing digit is! EIGHT
THE SECRET
This great trick relies on the power of 9.
After your friend has added up the digits and subtracted them, the answer will ALWAYS divide
by 9. If a number divides by nine, then when you add the digits up, they will also divide by 9.
If you check our example 7+8+3+1+8 = 27 which does divide by nine.
When your friend crosses a digit out, he then reads out the digits that are left. You add them up.
In the example we had 7+3+1+8 = 19
All you do now is see what you have to add on to your answer to get the next number that
divides by nine! The next number to divide by 9 after 19 is 27. So you need to add on EIGHT.
This is the number that was crossed out!
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