Seven machines at a certain factory operate at the same constant rate. If six of these machines, operating simultaneously, take 42 hours to fill a certain production order, how many fewer hours does it take all seven machines, operating simultaneously, to fill the same production order?  Choose the correct option:
A) 3  B) 4  C) 5  D) 6  E) 7
The total work is 6*42= 
<gadget id="calculator">6*42</gadget>
<output>252</output>
252 machine hours   The time required for seven machines is 252/7= 
<gadget id="calculator">252/7</gadget>
<output>36</output>
36 hours, thus 6 fewer hours.   The answer is 6.
<result>D</result>
Seven machines at a certain factory operate at the same constant rate. If six of these machines, operating simultaneously, take 42 hours to fill a certain production order, how many fewer hours does it take all seven machines, operating simultaneously, to fill the same production order?