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