When attempt to determine the validity of a bathroom scale by repeatedly placing 20, The systematic error is the mean value minus the true value which is 20.24 lb - 20.0 lb = 0.2 lb (I assume you know how to determine the mean). Systematic error is also called bias.
To find the maximum random error, the mean is subtracted from each value and the list of results is inspected for the largest absolute value. In this case, 20.6 lb is 0.4 lb greater than the mean and all other deviations are smaller, so 0.4 is the maximum random error.
