18
19
125 or -27 or -64 or 8 or 343 or 1
899
28
37
64
63
99,001
63
A digit is part of a number. the number 23 has two digits 2 and 3.
It is easiest to explain with an example. Suppose you have a number 4627. Then the number obtained by reversing [the order of] its digits is 7264. The difference between them is the bigger number minus the smaller number. So here it would be 7264 - 4627 = 2637.
The smallest two digit number is 00 (a number used as a wire gauge) and the largest is 99, so the difference between the largest and smallest 2-digits numbers is 99 (99 - 00)