Question

In an apartment complex with 28 units, 19 of the renters keep a pet. What percentage does not keep a pet?

Answer 1

If 19 out of 28 renters keep a pet, there are $28-19 = 9$ renters who don't keep a pet.

Whenever you have a subset of some set, and you want to know which percentage of the set the subset represents, you simply have to compute

$\dfrac{\text{number of elements in the subset}}{\text{number of elements in the set}}\times 100$

So, in your case, you're wondering what percentage of 28 does 9 represent. So, the formula becomes

$\dfrac{9}{28}\times 100 = 0.32\overline{142857}\times 100 = 32.\overline{142857} \approx 32.14\%$