x÷y=z
x:y=20:21
x:z=3:5
y=21x/20
z=5x/3
x÷(21x/20)=5x/3,20/21=5x/3,105x=60,x=4/7,y=3/5,z=20/21
被除数是4/7,除数是3/5.