| class | 0-10 | 10-20 | 20-30 | 30-40 | 40-50 |

And there respective frequency is given by,

| Frequency: | 3 | 5 | 8 | 3 | 1 |

Find the standard deviation.

Solution:

Class	$f_i$	$x_i$	$u_i$	$ui^2$	$fiui$	$fiui^2$
0-10	3	5	-2	4	-6	12
10-20	5	15	-1	1	5	5
20-30	8	25=A	0	0	0	0
30-40	3	35	1	1	3	3
40-50	1	45	2	4	4	4
	$\sum \mathrm{f}{\mathrm{i}}=$20				$\sum \mathrm{u}{\mathrm{i}} \mathrm{f}{\mathrm{i}}$= 6	$\sum …