Here are the steps:
1. Think of two digits, e.g. 3 and 5
2. Make the largest number: 53
3. Make the smallest number: 35
4. Find the difference: 53 - 35 = 18
I notice that the difference is always 18 no matter what two digits are chosen. This is because when forming the largest and smallest numbers from two digits, the ones digit remains the same while the tens digit changes. So the difference is always 10 * (tens digit of largest number - tens digit of smallest number) = 10 * (1 - 0) = 18.