设三个书架书分别为x,y,z
x-1/4x+1/3[z+1/6*(y+1/4x)]=5/6*(y+1/4x)=2/3*[z+1/6*(y+1/4x)]
x+y+z=90
55/72x+1/18y+1/3z=5/6y+5/24x,5x+3z=7y
5/6y+5/24x=2/3z+1/9y+1/36x
13/18y+13/72x=2/3z
52y+13x=48z
31x=20y,
x+y+z=90
3x+3y+3z=270
5x-7y+3z=0
10y-2x=270
20y-4x=540
31x-4x=540,x=20
y=31
z=90-31-20=39
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