Hello!
Here we could use something called the work equation.
If [tex]x[/tex] is considered proofing a chapter of the book, we can set up an equation such as, [tex]\frac{1}{9}+\frac{1}{5}=\frac{1}{x}[/tex].
Then, all we do is solve for it.
[tex]\frac{1}{9}+\frac{1}{5}=\frac{1}{x}[/tex]
[tex]5x+9x=45[/tex]
[tex]14x=45[/tex]
[tex]x=\frac{45}{14}[/tex]
So working together, it would take them [tex]\frac{45}{14}[/tex] hours to proof a chapter of the book.
Hope this helps!
