Explanation

Finish a work in 12 days a man working 8 hours per day
\therefore Finish a work in 1 day a man working =\left(8\times12\right) hours per day
\therefore Finish a work in 16 days a man working =\left(\frac{8\times12}{16}\right) hours per day
\rightarrow 6 hours per day.
Ans: 6 hours per day A man will have to work to finish the work.

Another way, Rules of Three Method

Days and Hours are Inversely Proportional.
\therefore\;16\;:\;12\;:\;:\;8\;:\;x
\rightarrow\large\frac{16}{12}=\frac{8}{x}
\rightarrow 16x=12\times8
\rightarrow x=\large\frac{12\times8}{16}
\rightarrow x=6 hours
Ans: He have to work 6 hours per day.

There’s another way to pose this same math question:

1. If a man working 8 hours per day takes 12 days to finish a task, how many hours per day must he work to complete the task in 16 days?
2. How many hours per day does a man need to work in order to finish a task in 16 days, if he typically works 8 hours per day and completes it in 12 days?
3. Given that a man completes a task in 12 days by working 8 hours per day, how many hours per day should he work to finish the same task in 16 days?
4. If a man completes a task in 12 days by working 8 hours per day, how many hours per day does he need to work to finish the same task in 16 days?