Answer
$f(x)=500(0.45)^{x}$
Work Step by Step
$\displaystyle \frac{225}{500}=0.45,\qquad \frac{101.25}{225}=0.45$
When $x$ increases by $1,\quad f(x)$ is multiplied by $0.45$
$\Rightarrow b=0.45$
$f(0)=Ab^{0}=500\Rightarrow \quad A=500$
Thus,
$f(x)=500(0.45)^{x}$