Answer
Answer: - $c$. Less than 5 hours.
Work Step by Step
The reason is very simple,
The second person alone can complete the job in 5 hours. So if he has help for his work, they both will complete the work in less than 5 hours.
We can check this too:
Work done by first person in one hour. $\frac{1}{7}$
Work done by second person in one hour. $\frac{1}{5}$
Let the work done $both$ of them in one hour = $\frac{1}{x}$
Thus, $\frac{1}{7}$ + $\frac{1}{5}$ = $\frac{1}{x}$
Thus, $\frac{5+7}{5(7)}$ = $\frac{1}{x}$
$\frac{12}{35}$ = $\frac{1}{x}$
Thus x = $\frac{35}{12}$ = 2$\frac{11}{12}$ hours.
