Let's use the relation:
[tex]\mathsf{\dfrac{1}{x} = \dfrac{1}{5} + \dfrac{1}{7}}\\ \\ \\ \mathsf{\dfrac{1}{x} = \dfrac{7+5}{35}}\\ \\ \\ \mathsf{\dfrac{1}{x} = \dfrac{12}{35}}\\ \\ \\ \mathsf{x = \dfrac{35}{12}}\\ \\ \\ \boxed{\mathsf{x\approx 2.92 \ h}}[/tex]
It would take about 3 hours to complete the work.
