Kenneth works in the shipping department of a toy factory that makes radio-controlled helicopters. Small helicopters weigh 3 pounds each, and are shipped in a container that weighs 15 pounds. Large ones, on the other hand, weigh 4 pounds apiece, and are shipped in a container that weighs 10 pounds. If these boxes can hold a certain number of helicopters each, all of the packed containers will have the same shipping weight. How many helicopters would fit in either container?