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.
