A survey was conducted by a group of students as a part of their environment awareness programme, in which they collected the following data regarding the number of plants in 20 houses in a locality. Find the mean number of plants per house. 

Number of plants	Number of houses
0-2	1
2-4	2
4-6	1
6-8	5
8-10	6
10-12	2
12-14	3

Which method did you use for finding the mean and why?

Number of plants	Number of houses (fi)	Class mark (xi)	fixi
0-2	1	1	1
2-4	2	3	6
4-6	1	5	5
6-8	5	7	35
8-10	6	9	54
10-12	2	11	22
12-14	3	13	39
Total	Σfi = 20		Σfixi = 162

Here, we have
                          = 162
Now,                  

Hence, the mean number of plants per house = 8.1
We have used the direct method for finding the mean because numerical values of x_iand f_i are small.

Consider the following distribution of daily wages of 50 workers of a factory:

Daily wages (in Rs.)	Number of workers
100-120	12
120-140	14
140-160	8
160-180	6
180-200	10

Find the mean daily wages of the workers of the factory by using an appropriate method.

The following distribution shows the daily pocket allowance of children of a locality. The mean pocket allowance is Rs. 18. Find the missing frequency 'f'.

Daily pocket allowance (in Rs.)	11-13	13-15	15-17	17-19	19-21	21-23	23-25
No. of childrens	7	6	9	13	f	5	4

Daily pocket allowance C.l.	No of children (fi)	Mid-value xi	fixi
11-13	7	12	84
13-15	6	14	84
15-17	9	16	144
17-19	13	18	234
19-21	f	20	20f
21-23	5	22	110
23-25	4	24	96
	Σfi = 44 + f		Σfixi = 752 + 20f

Here, we have                 
We know,                       
                                
                                18 (44 + f) = 752 + 20f
                                792 + 18f = 752 + 20f 
                                 18f - 20f  = 752 - 792 
                                          -2f = -40
                                             f = 20
Hence the Missing frequency is 20.

Thirty women were examined in a hospital by a doctor and the number of heart beats per minute were recorded and summarised as follows. Find the mean heart beats per minute for these women, choosing a suitable method.

Number of heart beats per minute	65-68	68-71	71-74	74-77	77-80	80-83	83-86
Number of women	2	4	3	8	7	4	2