You measure the width of a doorway and find that it is 1.20 m wide. Find the greatest possible error of your measurement

Given that the distance between two lines in the measurement instrument is 0.01m, the maximum error should be 0.01m/2 = 0.005 m.

If you do a good work, the real measure is 1.20m + / - 0.005m, this is between 1.195 and 1.205.