16.5% First, determine the total error by subtracting the accepted value from the value you measured. e = 17.7 - 21.2 = -3.5 Then divide the error by the accepted value. e = -3.5/21.2 = -0.165 = -16.5% Sometimes it's useful to keep track of if the error is positive, or negative. But for this case, we'll just go with the positive percentage. So the answer is 16.5% Key thing to remember. You divide by the CORRECT value to get the percentage.