Fifteen identical machines, running continuously at the same constant rate, take 10 days to complete a shipment. How many additional machines, each running at the same constant rate, would be needed to reduce the time required to complete a shipment by five days?  Pick one:
A) 2 B) 3 C) 4 D) 6 E) 9
From the question you know that 15R = 1/10. The question asks you (partially) to make the rate from 1/10 to 1/5 (drop from 10 day to 5). So the only thing that you need to do is to find the magic number than can convert 1/10 to 1/5.   So 1/10 * x = 1/5 (1 equation with one unknown). So by solving this you get x = 10/5 or 2/1. Thats it then! Take the magic number 4/3 and multiply BOTH sides of the original equation and you have:   12*(2/1)*R = (2/1) * 1/10   12 * 2 * R = 1/5, Hence 24R = 1/5, therefore 9 more machines!   9
<result>E</result>
Fifteen identical machines, running continuously at the same constant rate, take 10 days to complete a shipment. How many additional machines, each running at the same constant rate, would be needed to reduce the time required to complete a shipment by five days?