Time intervals measured by a clock give the following readings: 1 ....

First, calculate the mean of observations,&

$⇒ {A}_{m} = \dfrac{1 . 25 + 1 . 24 + 1 . 27 + 1 . 21 + 1 . 28}{5}= 1 . 25sec$,~
it means,& ∆ {A}_{1} = \left|1 . 25 - 1 . 25\right|s, ∆ {A}_{2} = \left|1 . 24 - 1 . 25\right|s, ∆ {A}_{3} = \left|1 . 27 - 1 . 25\right|s, ∆ {A}_{4} = \left|1 . 21 - 1 . 25\right|s & ∆ {A}_{5} = \left|1 . 28 - 1 . 25\right|s

~then calculates the mean of error,&

$∆ {A}_{m} = \dfrac{0 + 0 . 01 + 0 . 02 + 0 . 04 + 0 . 03}{5}= \dfrac{0 . 1}{5}$

$$~error& $\dfrac{∆ {A}_{m}}{Am} = \dfrac{0 . 1 \times 100}{5 \times 1 . 25} = 1 . 6$