y-(y-1/2)=3-(y+2/5)
y-y+1/2=3-y-2/5
y-y+y=3-2/5-1/2
y=2又1/10
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解：y-y+1/2=3-y-2/5
y=3-2/5 - 1/2
y=21/10
原式为：y-y+1/2=3-y-2/5
1/2=13/5-y
y=13/5-1/2=21/10
y-y+1/2=3-y-2/5
1/2-3+2/5=-y
y=3/5
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