If 9 workers can build 9 cars in 9 days, then how many days would it take 7 workers to build 7 cars?  Choose the correct answer:
A) 6
B) 7
C) 8
D) 9
E) 10
9 workers can build 1 car per day on average.   1 worker can build 1/9 of a car per day.   7 workers can build 7/9 car per day.   The time required to build 7 cars is 7 / (7/9)= 
<gadget id="calculator">7 / (7/9)</gadget>
<output>9</output>
9 days   The answer is 9.
<result>D</result>
If 9 workers can build 9 cars in 9 days, then how many days would it take 7 workers to build 7 cars?