Recall that percent error is given by the formula:
[tex]|\frac{real\text{ value-measured value}}{\text{real value}}|\cdot100[/tex]Therefore in our case this becomes:
[tex]|\frac{17-20}{17}|\cdot100=|\frac{3}{17}|\cdot100\approx17.65[/tex]Therefore the rounded (to two decimals) answer is : 17.65 %