The actual length of an object is 50 centimeters, but a student measures it as 47 centimeters. What is the percent error of the measurement?

Percent error is the size of the error divided by the actual value, times $100\%$: $\frac{|50-47|}{50}\cdot100\%=\frac{3}{50}\cdot100\%=6\%$, choice G. Choice H ($6.4\%$) divides the error by the measured value, $\frac{3}{47}$, but percent error always compares to the true value. Choice F ($3\%$) is the absolute error of $3$ centimeters, written as a percent without dividing by the actual value. Choice J ($94\%$) is $\frac{47}{50}$, the measurement as a percent of the actual length, not the error.