Answer
If the least expensive snack is bought, you can afford at most 15 snacks.
Work Step by Step
Step 1) If the most expensive sandwiches and drinks are ordered, the cost is: 3(7) + 3(2) = 27
Step 2) If the least expensive sandwiches and drinks are ordered, the cost is: 3(4) + 3(1) = 15
Step 3) If the total bill is $15 (least expensive sandwiches), we would still have 15 left, so we could buy 15 more snacks.