Question

Find the mean marks per student, using assumed-mean method:
$\begin{array}{ccccccc}\text{Marks}& 0-10& 10-20& 20-30& 30-40& 40-50& 50-60\\ \text{Number of students}& 12& 18& 27& 20& 17& 6\end{array}$

Answer 1

Class	Frequency (fi)	Mid values (xi)	Deviation (di) di = (xi – 25)	(fidi)
0 – 10	12	5	-20	-240
10 – 20	18	15	-10	-180
20 – 30	27	25 = A	0	0
30 – 40	20	35	10	200
40 – 50	17	45	20	340
50 – 60	6	55	30	180
Total	∑fi=100			∑(fi×di)=300

Let A = 25 be the assumed mean.

Then we have

$Mean=A+\frac{\sum f_i{d}_i}{\sum f_i}=25+\frac{300}{100}=28$