Working simultaneously and independently at an identical constant rate, 10 machines of a certain type can produce a total of x units of product P in 4 days. How many of these machines, working simultaneously and independently at this constant rate, can produce a total of 3x units of product P in 6 days?	Choose one.
A) 14  B) 15  C) 18  D) 20  E) 22
The rate of 10 machines is rate=job/time=x/4 units per day --&gt; the rate of 1 machine 1/10*(x/4)=x/40 units per day;   Now, again as {time}*{combined rate}={job done}   then 6*(m*x/40)=3x --&gt; m=20.   Answer: 20.
<result>D</result>
Working simultaneously and independently at an identical constant rate, 10 machines of a certain type can produce a total of x units of product P in 4 days. How many of these machines, working simultaneously and independently at this constant rate, can produce a total of 3x units of product P in 6 days?