Standard deviation

Range uses two values and IQR uses the middle half. Standard deviation uses every value and measures how far values typically lie from the mean.

The variance is in squared units. Take its positive square root to return to the data unit.

For 2, 4, 4 and 6, $n=4$, $\sum x=16$ and $\sum x^2=72$.

The mean is 4. Substituting before taking the square root gives the variance.

Values 5, 15 and 25 have frequencies 2, 5 and 3. Then $\sum f=10$, $\sum fx=160$ and $\sum fx^2=3050$.