Answer:
21 days
Step-by-step explanation:
This is an inverse proportion, since the number of days required will be less the more people there are.
let number of people be p and days be d, then the equation is
d = [tex]\frac{k}{p}[/tex] ← k is the constant of proportionality
to find k use the condition d = 28 when p = 9, hence
k = dp = 28 × 9 = 252
hence d = [tex]\frac{252}{p}[/tex]
when p = 12, then
d = [tex]\frac{252}{12}[/tex] = 21 days
