Richard's 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 30 hrs and a machine of type S does the same job in 15 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?  Options
A) 3	B) 4	C) 5	D) 9	E) 12
Rate of machine R =1/30   Rate of machine S =1/15   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/30 +x/15 =1/2   3x/30=1/2   x=30/6= 
<gadget id="calculator">30/6</gadget>
<output>5</output>
5   So no of machine R is 5   Answer 5
<result>C</result>
Richard's 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 30 hrs and a machine of type S does the same job in 15 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?