x:y=3:4
x=3y/4 (1)
y:z=4:5
z=5y/4 (2)
(1).(2)代入:x+y+z=36
3y/4+y+5y/4=36
3y=36
y=12
x=9
z=15
方程组的解:x=9,y=12,z=15