What are the five key numbers for a box and whisker plot, and what is the IQR? How do you calculate cumulative frequency? What's the relation between variance and std dev? What is MAD? Find the MAD fo

The frequency for 0-20 is 5.  
The frequency for 20-40 is 8.

$$
\text{Cumulative frequency} = 5 + 8 = 13
$$
The cumulative frequency for the class 20-40 is $\boxed{\text{13}}$.

$$
\bar{x} = \frac{5 + 10 + 15 + 20 + 25}{5} = \frac{75}{5} = 15
$$

$$
|5 - 15| = |-10| = 10
$$
$$
|10 - 15| = |-5| = 5
$$
$$
|15 - 15| = |0| = 0
$$
$$
|20 - 15| = |5| = 5
$$
$$
|25 - 15| = |10| = 10
$$

$$
\sum |x_i - \bar{x}| = 10 + 5 + 0 + 5 + 10 = 30
$$

$$
\text{MAD} = \frac{\sum |x_i - \bar{x}|}{n} = \frac{30}{5} = 6
$$
The mean absolute deviation (MAD) for the dataset is $\boxed{\text{6}}$.