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 Question from rejie, a student: if the digits are reverse , the number is 9 less than the original number.what is the equation?

Hi Rejie,

I expect that you are looking for a two digit number, and let's say digits are $a$ and $b$. In the first number $a$ is the tens digit and $b$ is the units digit and in the second number $b$ is the tens digit and $a$ is the units digit. Hence the first number is $10 a + b$ and the second number is $10 b + a.$