Split it in two number. The first one is formed by the two leftmost digits. The second one with the two right most digits
If your number is ABCD, you got AB and CD
Add the two numbers together
Substract the result from your original number
Split the result in two numbers. The first one is formed by the two leftmost digits. The second one with the two right most digits
If your result is ABCD, you got AB and CD
Add these two numbers together
You have found that the result is 99
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