18
19
125 or -27 or -64 or 8 or 343 or 1
899
28
37
64
99,001
63
63
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.
A digit is part of a number. the number 23 has two digits 2 and 3.
The two digits of the number 6 are simply 6 and 0, as it can be represented as 06. The difference between these two digits is 6 - 0, which equals 6. In essence, since there is only one non-zero digit in the number 6, the concept of "two digits" refers to its representation rather than actual distinct digits.