The last digit must be even, and only 3 of the even digits are such that
you can add another digit to them and get 13. These are 4, 6 and 8, because
0 and 2 are too small.
For each of the 3 ways we can choose a units digit, we can choose a tens digit
only 1 way, and that's whatever digit we must add to the units digit to get
13. For each of those, there are 9 ways to choose a hundreds digit, 1-9,
so as to have a three-digit number.
That's 3x1x9 = 27 ways. Here they all are:
1. 158
2. 176
3. 194
4. 258
5. 276
6. 294
7. 358
8. 376
9. 394
10. 458
11. 476
12. 494
13. 558
14. 576
15. 594
16. 658
17. 676
18. 694
19. 758
20. 776
21. 794
22. 858
23. 876
24. 894
25. 958
26. 976
27. 994
Edwin
