A class teacher has the following absentee record of 40 students of a class for the whole term.

To find the class mark of each interval, the following relation is used.
xᵢ = (Upper Limit + Lower Limit)/2
Taking 17 aa assumed mean (a), dᵢ and fᵢdᵢ are calculated as follow.

From the table, we obtain
∑fᵢ = 40 + f, ∑fᵢdᵢ = -181 and a = 17
mean (X̄) = a + (∑fᵢdᵢ / ∑fᵢ) = 17 + (-181/40) = 17 – 4.525 = 12.475 = 12.48
Therefore, the mean number of days is 12.48 days for which a student was absent.