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?
Options:
A) 11
B) 13
C) 15
D) 18
E) 20

Rationale: "the rate of 5 machines is rate = job / time = x / 4 units per day - - > 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 ) = 2 x - - > m = 20 . answer : e ."
Answer: E