In Ancient times, ceramic amphorae were used for the transport and storage of various products, both liquid and dry.

In total, a rich father gives to his three sons: 15 amphorae with gold sand, 15 half-empty amphorae with gold sand and 15 empty amphorae.

They want that each brother gets the same amount of gold sand and the same number of amphorae, without transferring the gold sand from any amphora to any other amphora.

What is the least number of empty amphorae a brother can get?