题目内容

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（　　）days to work together to finish the work．
A、10B、12C、15D、50
分析：设他们一起工作x天可结束工作，根据工作量=工作时间×工作效率，完成工作工作量为1，根据题意可列方程求解．
解答：解：设他们一起工作x天可结束工作．
（
1
20
+
1
30
）x=1
x=12
需要12天可完成工作．
故选B．
点评：本题考查理解题意能力，关键是知道工作量=工作时间×工作效率，完成工作工作量为1，从而列方程可得解．
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