问题
选择题
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．10
B．12
C．15
D．50
答案
设他们一起工作x天可结束工作．
（
+1 20
）x=11 30
x=12
需要12天可完成工作．
故选B．