Answer
It takes them $\frac{20}{9}$ days.
Work Step by Step
Rate is given by jobs completed divided by the time it took to do those jobs.
Mr. Dodsons rate →$R_{D}$
Mr. Dodson's son's rate →$R_{S}$
How long it takes to paint the house→J
Mr.Dodson can paint a house by himself in 4 days so his rate is $\frac{1}{4}$.
His son can paint a house by himself in 5 days so his rate is $\frac{1}{5}$.
$R_{D}$T+$R_{S}$T=J
$\frac{1}{4}$T+$\frac{1}{5}$T=1
$\frac{9}{20}$T=1
T=$\frac{20}{9}$
