鸡2元/只, 鸭4元/只,鹅5元/只设一只鸡x元,一只鸭y元,一只鹅z元,依题意3x+y=2z,x+2y+2z=20.解得x=6z/5-4,y=12-8z/5,x,y,z都是非负整数,∴z=5,x=2,y=4.