Six machines at a certain factory operate at the same constant rate. If five of these machines, operating simultaneously, take 42 hours to fill a certain production order, how many fewer hours does it take all six machines, operating simultaneously, to fill the same production order?
Pick one
A) 4  B) 6  C) 7  D) 9  E) 12
The total work is 5*42= 
<gadget id="calculator">5*42</gadget>
<output>210</output>
210 machine hours   The time required for six machines is  210/6= 
<gadget id="calculator"> 210/6</gadget>
<output>35</output>
35 hours, thus 7 fewer hours.   The answer is 7.
<result>C</result>
Six machines at a certain factory operate at the same constant rate. If five of these machines, operating simultaneously, take 42 hours to fill a certain production order, how many fewer hours does it take all six machines, operating simultaneously, to fill the same production order?