Find the mean deviation about the mean of the distribution:

$\text{Size}$	$20$	$21$	$22$	$23$	$24$
$\text{Frequency}$	$6$	$4$	$5$	$1$	$4$

First,calculate the mean $\bar{x}$:
$\bar{x} = \frac{\Sigma f_i x_i}{\Sigma f_i} = \frac{(20 \times 6) + (21 \times 4) + (22 \times 5) + (23 \times 1) + (24 \times 4)}{6 + 4 + 5 + 1 + 4} = \frac{120 + 84 + 110 + 23 + 96}{20} = \frac{433}{20} = 21.65$
Next,calculate the mean deviation about the mean:

$\text{Size } (x_i)$	$\text{Freq } (f_i)$	$|x_i - \bar{x}|$	$f_i |x_i - \bar{x}|$
$20$	$6$	$1.65$	$9.90$
$21$	$4$	$0.65$	$2.60$
$22$	$5$	$0.35$	$1.75$
$23$	$1$	$1.35$	$1.35$
$24$	$4$	$2.35$	$9.40$
$\text{Total}$	$20$	-	$25.00$

$\text{Mean Deviation} = \frac{\Sigma f_i |x_i - \bar{x}|}{\Sigma f_i} = \frac{25}{20} = 1.25$