153403

Part 1:
I looked at all the combinations of the digits 2, 5, 7 and 8 and crossed out all the combinations where the first tens digit was smaller than the second tens digit; these answers would be negative. This gave me:
29, 31, 14, 47, 26, 53, 7, 25, 13, 58, 62 and 35.
Part 2:
The simplest idea I could think of to start with, was to use consecutive numbers. I realised that it didn't matter what consecutive numbers I used because the differences would cancel out. For example,
using 1, 2, 3 and 4, 43 - 21 = 22. Using 5, 6, 7 and 8, 87 - 65 = 22.
So when I tried 1, 2, 3, and 4, and crossed out all the negative answers, I got: 7, 9, 11, 13, 18, 22, 29 and 31.
Part 3:
By the time I'd answered Part 2, I knew how to show the answer to Part 3.