I would set up a proportion:
[tex]\frac{20}{x}=\frac{5}{4}[/tex]
The proportion basically says, "if there are 5 kids meals for every 4 adult meals, that means 20 kids meals would mean x adult meals." Just cross multiply and then solve for x:
[tex]\frac{20}{x}=\frac{5}{4}\\5x=80\\x=16[/tex]
That means there are 20 kids meals sold and 16 adult meals sold, so the result is:
36 meals in total.