The task can be finished in 12 days for 3 workers. For this condition that the work can be done by 3 workers in 12 days, we have
[tex]task=12days\times3workers=36\text{ worker-days}[/tex]
If we have 9 working day to complete the task, we will be needing
[tex]\frac{36\text{ worker days}}{9\text{ days}}=4\text{ workers}[/tex]It takes 4 workers to finish the same task in 9 days.
