1. Save 9 1 dollar bills.
2. Use them to buy your 9 snacks. (Not at the same time, though!)
3. Collect all the change the vending machine gives you.
4. You should have 90 cents in dimes if you used the right kind of vending machine. (The kind where everything costs 90 cents)
5. If you have anything less than 90 cents, keep spending money until you have 90 cents.
6. When you have at least 90 cents, put it into the vending machine and buy yourself the eleventh thing!