Question

Kim borrowed money at an annual simple
interest rate of 6% to buy a car. How much did
she borrow if she paid $4200 in interest over
the life of the 4-year loan? Solve the equation
I = Prt for P and justify each step.

Answer 1

$\bf ~~~~~~ \textit{Simple Interest Earned} \\\\ I = Prt\qquad \begin{cases} I=\textit{interest earned}\dotfill & \ 4200\\ P=\textit{original amount deposited}\\ r=rate\to 6\%\to \frac{6}{100}\dotfill &0.06\\ t=years\dotfill &4 \end{cases} \\\\\\ 4200=P(0.06)(4)\implies 4200=0.24P\implies \cfrac{4200}{0.24}=P\implies 17500=P$