Can you explain how to solve this math problem: A three-ounce box of flavored jello makes 10 small jello cups.  Greg wants to make small jello cups for his son's outdoor birthday party.  There will be 30 kids and he wants to have enough so that each kid can have 4 jello cups.  Jello is currently on sale for $1.25.  How much will he spend on jello?