Working simultaneously and independently at an identical constant rate, 6 machines of a certain type can produce a total of x units of product P in 5 days. How many of these machines, working simultaneously and independently at this constant rate, can produce a total of 4x units of product P in 10 days? Choose the correct choice from the following:
A) 9
B) 10
C) 12
D) 14
E) 18
The rate of 6 machines is rate=job/time=x/5 units per day --&gt; the rate of 1 machine 1/6*(x/5)=x/30 units per day;   Now, again as {time}*{combined rate}={job done}   then 10*(m*x/30)=4x --&gt; m=12.   Answer: 12.
<result>C</result>
Working simultaneously and independently at an identical constant rate, 6 machines of a certain type can produce a total of x units of product P in 5 days. How many of these machines, working simultaneously and independently at this constant rate, can produce a total of 4x units of product P in 10 days?