23. The distance between two cities in kilometers is expressed as a two-digit number, where the left digit is equal to the difference between this number and the number written with the same digits but in reverse order. What is this distance?
23. Let the desired number be $10 a+b$. Then

$$
a=(10 a+b)-(10 b+a), a=9 a-9 b, 9 b=8 a
$$

from which $a=9, b=8$. Answer: 98 km.