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- Thread starter Zaynab
- Start date

Let your 2 digit number be "xy".

The number is actually 10x + y (tens and units, back to primary school )

Add the digits together = x+y

Take the sum (x+y) from the number (10x+y)

= 10x+y -(x+y)

= 10x -x +y -y

= 9x

so the answer is always 9 times the first digit of your number, so it will always be a multiple of 9. Look at all the symbols for the 9 times table....