Question

Calculate the arithmetic mean from the following series by direct method:

Marks 

5-15

15-25

25-35

35-45

45-55

No. of students

8

12

15

9

6

Answer 1

$$10\times8=80$$

$$20\times12=240$$

$$30\times15=450$$

$$40\times9=360$$

$$50\times6=300$$

$$\frac{80+240+450+360+300}{8+12+15+9+6}=28.6$$