If Nate paints the wall in 4 hours, it means that he can paint 1/4 of the wall every hour.
Similarly, if Jill paints the wall in 7 hours, it means that he can paint 1/7 of the wall in one hour.
So, together, they paint the following fraction of the wall in one hour:
[tex] \frac{1}{4}+\frac{1}{7} = \frac{4+7}{28} = \frac{11}{28} [/tex].
At this rate, they need 28/11 hours to paint the whole wall, in fact, after that amount of time they paint
[tex] \frac{11}{28}\frac{28}{11} = 1 [/tex]
i.e. the whole wall
