The formula for simple interest is <em>I</em> = <em>prt</em>, where <em>I</em> is the amount of interest, <em>p</em> is the principal borrowed, <em>r</em> is the interest rate written as a decimal number, and <em>t</em> is the amount of time in years. First we find the amount of interest. He borrowed $35000 but paid back $46375. That means he paid 46375-35000 = $11375 in interest. We can now substitute our information into our interest formula: 11375=35000(<em>r</em>)(5) 11375=35000(5)(<em>r</em>) ----- remember that multiplication is commutative 11375=175000<em>r</em> Divide both sides by 175000 to cancel it: 11375/175000 = 175000<em>r</em>/175000 0.065 = <em>r</em> To convert this to a percentage, we multiply by 100: 0.065(100) = 6.5%