Working simultaneously and independently at an identical constant rate, 5 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 2x units of product P in 2 days?  Choose the correct choice from the following:
A) 11  B) 13  C) 15  D) 18  E) 20
The rate of 5 machines is rate=job/time=x/4 units per day --&gt; the rate of 1 machine 1/5*(x/4)=x/20 units per day;   Now, again as {time}*{combined rate}={job done}   then 2*(m*x/20)=2x --&gt; m=20.   Answer: 20.
<result>E</result>
Working simultaneously and independently at an identical constant rate, 5 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 2x units of product P in 2 days?