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

First, calculate the mean of observations, 

$\Rightarrow A_m=\frac{1.25+1.24+1.27+1.21+1.28}{5}$$=1.25 \sec$, 
it means, $$\begin{gathered} ∆A_1=\left|1.25-1.25\right| \mathrm{s}, ∆A_2=\left|1.24-1.25\right| \mathrm{s},∆A_3=\left|1.27-1.25\right| \mathrm{s}, \\ ∆A_4=\left|1.21-1.25\right| \mathrm{s} \& ∆A_5=\left|1.28-1.25\right| \mathrm{s} \end{gathered}$$

 then calculates the mean of error, 

$∆A_m=\frac{0+0.01+0.02+0.04+0.03}{5}$$=\frac{0.1}{5}$

$\%$ error $\frac{∆A_m}{A_m}=\frac{0.1\times 100}{5\times 1.25}=1.6\%$