Answer: The largest you can't make is 7 cents.
Step-by-step explanation:
Once you get 3 consecutive amounts, you can get all larger ones just by adding 3 cent stamps. So let's see what you can make:
1 no
2 no
3 yes
4 no
5 yes
6 yes
7 no
8 yes, and this will give you 11 14 17 etc
9 yes, and this will give you 12 15 18 etc
10 yes, and this will give you 13 16 19 etc.
