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:     0-2    2-4    4-6    6-8    8-10    10-12   12-14 Number of houses:     1       2        1       5        6           2          3 Which method did you use for finding the mean, and why?