Time intervals measured by a clock give the following readings : 
1.25 s, 1.24 s, 1.27 s, 1.21 s and 1.28 s 
What is the percentage relative error of the observations?

The arithmetic mean of given values is taken as true value.

$\begin{array}{l}{\mathrm{t}}_{\text{mean }}=\frac{{\mathrm{t}}_1+{\mathrm{t}}_2+{\mathrm{t}}_3+{\mathrm{t}}_4+{\mathrm{t}}_5}{5}\\ {\mathrm{t}}_{\text{mean }}=\frac{1.25+1.24+1.27+1.21+1.28}{5}\\ {\mathrm{t}}_{\text{mean }}=1.25\mathrm{s}\\ {\mathrm{\Delta t}}_{\text{mean }}=\frac{\left|{\mathrm{\Delta t}}_1\right|+\left|{\mathrm{\Delta t}}_2\right|+\left|{\mathrm{\Delta t}}_3\right|+\left|{\mathrm{\Delta t}}_4\right|+\left|{\mathrm{\Delta t}}_5\right|}{5}\end{array}$

$\begin{array}{l}{\mathrm{\Delta t}}_{\text{mean }}=\frac{0+0.01+0.02+0.04+0.03}{5}=0.02\mathrm{s}\\ \text{ Percentage error }=\frac{{\mathrm{\Delta t}}_{\text{mean }}}{{\mathrm{t}}_{\text{mean }}}\times 100=\frac{0.02}{1.25}\times 100\\ =1.6\mathrm{\%}\end{array}$