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 18 hrs and a machine of type S does the same job in 9 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?  Choose the correct choice:
A) A)3
B) B)4
C) C)6
D) D)9
E) E)12
Rate of machine R =1/18   Rate of machine S =1/9   since same no of machines used for R and S to do the same work in 2 hrs   So collective rate needed to finish the work in 2 hrs= 1/2   Let the no of machine be x   So, x/18 +x/9 =1/2   3x/18=1/2   x=18/6= 
<gadget id="calculator">18/6</gadget>
<output>3</output>
3   So no of machine R is 3   3nswer 3
<result>A</result>
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 18 hrs and a machine of type S does the same job in 9 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?