x/(120-x)=(60-x)/60

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x/(120-x)=(60-x)/60

x/(120-x)=(60-x)/60
x/(120-x)=(60-x)/60

x/(120-x)=(60-x)/60
两边都乘以 60(120-x) (x≠120)

60x=(60-x)(120-x)
60x=7200-180x+x²
x²-240x+7200=0
(x-120)²=14400-7200
=7200
x-120=±√7200=±60√2
x=120±60√2