Damian measured a line to be 9.4 inches long. If the actual length of the line is 7.8 inches, then what was the percent error of the measurement, to the nearest tenth of a percent?
The percent error is equal to the absolute difference between the measured and actual values divided by the actual value, multiplied by 100%.
The absolute difference is $|(9.4-7.8)|=1.6$.
The percent error is $\frac{1.6}{7.8}\cdot100\%=20.5\%$.
To the nearest tenth of a percent, the percent error is $\boxed{20.5\%}$.