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 invested, <em>r</em> is the rate as a decimal number and <em>t</em> is the amount of time. Substituting our given information we have 128 = <em>p</em>(0.016)(2) ----- 1.6% = 1.6/100 = 0.016 128 = <em>p</em>(0.032) Divide both sides by 0.032: 128/0.032 = <em>p</em>(0.032)/0.032 4000 = <em>p</em> He needs to invest $4000.