The minimum that can be measured with a scale is 1 mm. If the length of an object is 1 mm, what will be the percentage relative error of the measurement?

Correct answer: d 50%

Absolute error = half of 1 mm = 0.5 mm; relative error = 0.5 ÷ 1 × 100% = 50%.

Why the other options are wrong

• a 0.5%: 0.5 mm is the absolute error, not a percentage; relative error = 0.5 ÷ 1 = 0.5, that is 50%.
• b 5%: 5% would result from a 0.5 mm error on a 10 mm length; here the length is 1 mm.
• c 15%: 15% is wrong; with absolute error 0.5 mm and length 1 mm the ratio is 0.5, that is 50%.