设公鸡x只,母鸡y只,小鸡z只
10x+5y+0.5z=100.(1)
x+y+z=100.(2)
(2)乘0.5得0.5x+0.5y+0.5z=50.(3)
(1)-(3)=9.5x+4.5y=50.(4)
y=100-19x/9
(2)乘5得5x+5y+5z=500.(5)
(5)-(1)=-5x+4.5z=400
z=800+10x/9
∴x+y+z=x+100-19x/9+800+10x/9=100
得x=1,y=9,z=90
∴公鸡1只,母鸡9只,小鸡90只