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 21 hrs and a machine of type S does the same job in 42 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) 6	B) 7	C) 8	D) 9	E) 10
Type R completes 1/21 of the job each hour.   Type S completes 1/42 of the job each hour.   Together R and S complete 1/21 +1/42 = 1/14 of the job each hour.   Let n be the number of each type of machine.   2*n*(1/14) = 1 job completed   n = 7   The answer is 7.
<result>B</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 21 hrs and a machine of type S does the same job in 42 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?