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 themdays to work together to finish the work．

A.
10
B.
12
C.
15
D.
50

B
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：设他们一起工作x天可结束工作．
（ $\text{[math]}$ + $\text{[math]}$ ）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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