[tex] \bf ~~~~~~ \textit{Simple Interest Earned}\\\\I = Prt\qquad \begin{cases}I=\textit{interest earned}\\P=\textit{original amount deposited}\dotfill & \$400\\r=rate\to 3.5\%\to \frac{3.5}{100}\dotfill &0.035\\t=years\dotfill &3\end{cases}\\\\\\I=(400)(0.035)(3)\implies I=42 [/tex]
keeping in mind that whatever% of anything is just (whatever/100) * anything.
the amount borrowed is $400, what is 3.5% of 400? well is just (3.5/100) * 400, or 14.
so after 1 year, you owe your parents (3.5/100) * 400, or 14.
after 2 years, you owe them [ (3.5/100) * 400] + [ (3.5/100) * 400] , or 14 + 14.
after 3 years, you owe them, [ (3.5/100) * 400] + [ (3.5/100) * 400] + [ (3.5/100) * 400], or 14 + 14 + 14 = 42.
