Answer
(a) Independent variable - $a$; Dependent variable - $P$
(b) There were $231.427$ million Americans who were $20$ years or older in $2012$.
(c) There were $327.287$million Americans in $2012$.
Work Step by Step
$P(a)=0.014a^2 - 5.073a + 327.287$
a.
Independent variable - $a$
Dependent variable - $P$
b. Substitute $20$ to $a$. This will give the number of Americans who were $20$ years or older in $2012$.
$P(a)=0.014a^2 - 5.073a + 327.287$
$P(20)=0.014(20)^2 - 5.073(20) + 327.287$
$P(20)=231.427$
Therefore, there were $231.427$ million Americans that were $20$ years of age or older in $2012$.
c. Substitute $0$ to $a$. This will give the number of Americans who were $0$ years or older (the entire population) in $2012$.
$P(a)=0.014a^2 - 5.073a + 327.287$
$P(0)=0.014(0)^2 - 5.073(0) + 327.287$
$P(0)=327.287$
Therefore, there were $327.287$ million Americans in 2012.
