Clever but very simple. When two numbers containing the same digits are
subtracted from one another the result is always divisible by 9 which also
means that adding all the digits in the result together will also make a
number divisible by 9. It's then quite easy to work out which number was
removed.


e.g. 3145 - 1453 = 1692
Remove the 6, leaving 192
Add digits of 192 together = 12
Add digits of 12 together = 3
Subtract 3 from 9 = 6 which is the missing number.

Yes... I am a nerd and got high marks in maths at school

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