Four machines, each working at the same constant rate, together can complete a certain job in 20 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 10 days?
Choose the correct choice:
A) 5 B) 3 C) 4 D) 6 E) 1
Rate of one machine = 1 job / (20*4) days   let X = number of machines needed to complete the job in 10 days   1/(4*20) * 10 * X = 1 job   X = 8   8-4= 
<gadget id="calculator">8-4</gadget>
<output>4</output>
4   ANSWER: 4
<result>C</result>
Four machines, each working at the same constant rate, together can complete a certain job in 20 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 10 days?