Answer
$ 13.33\%$
Work Step by Step
Apply the Simple Interest Formula Theorem
If a principal of $P$ dollars is borrowed for a period of $t$ years
at a per annum interest rate $r,$ expressed as a decimal,
the interest $I$ charged is $I=Prt$
---
$P={{\$}} 5000 $
$t= 9$ months = $\displaystyle \frac{9}{12}=0.75$ years
$P+I={{\$}} 5500\Rightarrow I=500$
Find $r$ from $I=Prt \displaystyle \Rightarrow r=\frac{I}{Pt}$
$ r=\displaystyle \frac{500}{(5000)(0.75)}\approx 0.1333=13.33\%$
