Answer:
12 hours
Step-by-step explanation:
Let x hours be the time needed for the experienced employee, then 3x hours is time needed for the new employee.
Then, the experienced employee completes [tex]\dfrac{1}{x}[/tex] of the whole work per hour and the new employee completes [tex]\dfrac{1}{3x}[/tex] of the whole work per hour.
Altogether the can complete
[tex]\dfrac{1}{x}+\dfrac{1}{3x}=\dfrac{4}{3x}[/tex]
of the whole work per hour and need
[tex]\dfrac{1}{\frac{4}{3x}}=\dfrac{3x}{4}[/tex]
hours to finish the whole work.
Working together, it takes them 9 hours, then
[tex]\dfrac{3x}{4}=9\\ \\3x=36\\ \\x=12\ hours[/tex]
