PREP1-PS-89

byct1

89.10086-!-item-!-187;#058&007098

A company has two types of machines, type R and type S.Operating at a constant rate, a machine of type R does a certain job in 36 hours and a machine of type S does the same job in 18 hours.If the company used the same number of each type of machine to do the job in 2 hours, how many machines of type R were used?

(A) 3

(B) 4

(C) 6

(D) 9

(E) 12

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【答案】c

这题，不是时间乘以速度吗？我就设2M 为总的，然后2×（1/36 +1/18）=2M
于是就错了。。这种题我老错

feel1979

n为机器数量,设v为R的效率
那么总的工作量为2*(nv+2nv)=36v
n=6

byct1

有人知道吗？？