题目:
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 in36 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?
选项:
A、3
B、4
C、6
D、9
E、12
答案:
C