(Q16). A class teacher has the following attendance record of 40 students of a class for the whole term. Find the mean number of days a student was present out of 56 days in the term.

Number of days 35-38 38-41 41-44 44-47 47-50 50-53 53-56
Number of students 1 3 4 4 7 10 11

Solution :

Number of days Number of students (fi) Class marks (xi) di = xi - a, a = 51.5 fidi
35-38 1 36.5 - -
38-41 3 39.5 - -
41-44 4 42.5 - -
44-47 4 45.5 - -
47-50 7 48.5 - -
50-53 10 51.5 a
53-56 11 54.5
- ∑fi = 40 - - ∑fidi = -

Here, a = 51.5

x̄ = a +
∑fidi
∑fi
 = 51.5 -
40
 = .025 ≃ days.