If the tens digit is 1, twice 1 is 2 and the digits that are greater than
2 are 3,4,5,6,7,8,9. So that's 7 ways.
If the tens digit is 2, twice 2 is 4 and the digits that are greater than
4 are 5,6,7,8,9. So that's 5 ways.
If the tens digit is 3, twice 3 is 6 and the digits that are greater than
2 are 7,8,9. So that's 3 ways.
If the tens digit is 4, twice 4 is 8 and the only digit greater than
8 is 9. So that's 1 ways.
The tens digit cannot be 5 or larger, since twice 5 is not a digit.
So that 7+5+3+1 = 16 of them. Might as well list them all:
1. 13
2. 14
3. 15
4. 16
5. 17
6. 18
7. 19
8. 25
9. 26
10. 27
11. 28
12. 29
13. 37
14. 38
15. 39
16. 49
Edwin
