lets assume that my number has 2 digits: x and y, x is the tens digit, y the ones digit

x and y are elements of {1, 2, 3, 4, 5, 6, 7, 8, 9}

My number has a tens digit that is 8 more than the ones digit means

x = 8 + y

x <= 9

8 + y <= 9

y <= 9 - 8

y <= 1

now we have y>0 and y<=1 follows y = 1

x = 8 + 1

x = 9

mu number is 91.