1/X+1/y=1/20 (1)
1/Y+1/Z=1/30 (2)
3/X=1.5×5/Z (3)
(1)-(2)得 1/x-1/z=1/60 (4)
由(3)得 2/x-5/z=0 (5)
(4)*2-(5)得 3/z=1/30
z=90
把z=90代入（4）得 x=36
把z=90代入（2）得 y=45
∴x=36,y=45,z=90
1/X+1/y=1/20 1/Y+1/Z=1/30 3/X=1.
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