Answer:
Part A)
About 0.51% per year.
Part B)
About 0.30% per year.
Part C)
About 28.26%.
Step-by-step explanation:
We are given that the population of Americans age 55 and older as a percentange of the total population is approximated by the function:

Where <em>t</em> is measured in years with <em>t</em> = 0 being the year 2000.
Part A)
Recall that the rate of change of a function at a point is given by its derivative. Thus, find the derivative of our function:
![\displaystyle f'(t) = \frac{d}{dt} \left[ 10.72\left(0.9t+10\right)^{0.3}\right]](https://tex.z-dn.net/?f=%5Cdisplaystyle%20f%27%28t%29%20%20%3D%20%5Cfrac%7Bd%7D%7Bdt%7D%20%5Cleft%5B%2010.72%5Cleft%280.9t%2B10%5Cright%29%5E%7B0.3%7D%5Cright%5D)
Rewrite:
![\displaystyle f'(t) = 10.72\frac{d}{dt} \left[(0.9t+10)^{0.3}\right]](https://tex.z-dn.net/?f=%5Cdisplaystyle%20f%27%28t%29%20%3D%2010.72%5Cfrac%7Bd%7D%7Bdt%7D%20%5Cleft%5B%280.9t%2B10%29%5E%7B0.3%7D%5Cright%5D)
We can use the chain rule. Recall that:
![\displaystyle \frac{d}{dx} [u(v(x))] = u'(v(x)) \cdot v'(x)](https://tex.z-dn.net/?f=%5Cdisplaystyle%20%5Cfrac%7Bd%7D%7Bdx%7D%20%5Bu%28v%28x%29%29%5D%20%3D%20u%27%28v%28x%29%29%20%5Ccdot%20v%27%28x%29)
Let:

Then from the Power Rule:

Thus:
![\displaystyle \frac{d}{dt}\left[(0.9t+10)^{0.3}\right]= 0.3(0.9t+10)^{-0.7}\cdot 0.9](https://tex.z-dn.net/?f=%5Cdisplaystyle%20%5Cfrac%7Bd%7D%7Bdt%7D%5Cleft%5B%280.9t%2B10%29%5E%7B0.3%7D%5Cright%5D%3D%200.3%280.9t%2B10%29%5E%7B-0.7%7D%5Ccdot%200.9)
Substitute:

And simplify:

For 2002, <em>t</em> = 2. Then the rate at which the percentage is changing will be:

Contextually, this means the percentage is increasing by about 0.51% per year.
Part B)
Evaluate f'(t) when <em>t</em> = 17. This yields:

Contextually, this means the percetange is increasing by about 0.30% per year.
Part C)
For this question, we will simply use the original function since it outputs the percentage of the American population 55 and older. Thus, evaluate f(t) when <em>t</em> = 17:

So, about 28.26% of the American population in 2017 are age 55 and older.