If 48 machine can finish a job in 80 days, then how many more machines would be needed to finish the job in one -fifth less time?  Choices:
A) a. 4  B) b. 8  C) c. 10  D) d. 12  E) e. 16
You might think of this in a management context - we can use the principle of 'person-hours' to solve any problem where we have identical workers. So, using simpler numbers, suppose you know that 6 identical employees, working simultaneously, would finish a job in 5 hours. Then that job requires 6*5= 
<gadget id="calculator">6*5</gadget>
<output>30</output>
30 total hours of person-work. If instead you wanted the job done in 3 hours, you'd assign   30/3= 
<gadget id="calculator">  30/3</gadget>
<output>10</output>
10 employees to do the job, because you want to get a total of 30 hours of work from the employees.   We can solve this problem identically. If 48 machines (identical ones, I assume) work simultaneously for 80 days, they will do a total of 48*80 machine-days of work. So the job requires 48*80 days of machine work in total. We instead want the job done in 1/5 less time, so in 64 days. So we'll need 48*80/64= 
<gadget id="calculator">48*80/64</gadget>
<output>60</output>
60 machines, or 12 additional machines.   d. 12
<result>D</result>
If 48 machine can finish a job in 80 days, then how many more machines would be needed to finish the job in one -fifth less time?