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There is a piece of work．It takes Mr．A alone 20 days to finish，and Mr．B 30 alone days to finish．It takes them（　�。ヾays to work together to finish the work．
A．10B．12C．15D．50
設他們一起工作x天可結束工作．
（
1
20
+
1
30
）x=1
x=12
需要12天可完成工作．
故選B．
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