Solution:

Mean value \(\bar{t} = (1.25+1.24+1.27+1.21+1.28)/5 = 1.25\text{ s}\). Mean absolute error \(\delta\bar{t} = (|0|+|0.01|+|0.02|+|0.04|+|0.03|)/5 = 0.1/5 = 0.02\text{ s}\). Percentage relative error \( = (\Delta\bar{t} / \bar{t}) \times 100% = (0.02 / 1.25) \times 100% = 1.6%\).