In a two digit number, the tens digit is t and its units digit is u. If the digits are reversed this new number is 9 less than the original number.

Answer 1

this will happen if t = u+1, as in
54 = 45 + 9
76 = 67 + 9
To see why,
10u+t = 10t+u - 9
9u = 9t - 9
u = t-1

Answer 2

Thank you, oobleck