Find the ratio $\dfrac{a}{b}$ if $\dfrac{a+b}{a-3b}=5$.

Simplify as follows:

 ${a+b} = 5 (a-3b)$

$a + b = 5a - 15b$

$b = 4a - 15b$

$16b = 4a$

$4b = a$

${\color{brown}\boxed{4}} = {a \over b}$