899
It is called the range.
47 - 31 = 16
If the number with the digits reversed can have a leading 0 so that it is a 1-digit number, then 16. Otherwise 13.
1
> 9999876 Actually, 9999876 has four different digits (6, 7, 8, 9). The largest 7-digit number with three different digits is 9999987.
99,001
28
37
987654321 is the highest nine-digit number with no repeated digits.
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)
19
31
63
99