Question

Hilaria borrowed $8,000 from her grandfather to pay for college. Five years later, she paid him back the $8,000, plus $1,600 interest. What was the rate of interest (as a percent)? Solve using the simple interest formula.

Answer 1

`~~~~~~ \textit{Simple Interest Earned} \\\\ I = Prt\qquad \begin{cases} I=\textit{interest earned}\dotfill&\$1600\\ P=\textit{original amount deposited}\dotfill & \$8000\\ r=rate\to r\%\to (r)/(100)\\ t=years\dotfill &5 \end{cases} \\\\\\ 1600=(8000)\left( (r)/(100) \right)(5)\implies \cfrac{1600}{(8000)(5)}=\cfrac{r}{100} \\\\\\ \cfrac{1}{25}=\cfrac{r}{100}\implies \cfrac{100}{25}=r\implies \boxed{\stackrel{\%}{4}=r}`