Sports Depot sells tennis balls in cans of either 3 or 4 tennis balls each. What is the greatest number of tennis balls that Kaycee cannot buy from Sports Depot as an exact combination of these two types of cans?
After that you can always put a pack of 4 into the cart and take out a pack of 3 resulting in the addition of 1 tennis ball OR (if there are no packs of 3 left in the cart) take out two packs of 4 and put in 3 packs of 3 resulting in the addition of 1 tennis ball.
