It can be observed that class intervals are not continuous. There is a gap of 1 between two class intervals. Therefore, 1/2 [0.5) has to be added to the upper-class limit and 1/2 (0.5) has to be subtracted from the lower-class limit of each interval. Class mark (xᵢ) can be obtained by using the following relation.
xᵢ = (Upper Limit + Lower Limit)/2
Class size (h) of this data = 3. Taking 57 as assumed mean (a), dᵢ, uᵢ, fᵢuᵢ are calculated as follows.

It can be observed that
∑fᵢ = 400, ∑fᵢuᵢ = 25, a = 57 and h = 3
mean (X̄) = a + (∑fᵢuᵢ / ∑fᵢ)h = 57 + (25/400) × 3 = 57 + 3/16 = 57 + 0.1875 = 57.19
Mean number of mangoes kept in a packing box is 57.19.
Set deviation method is used here as the values of fᵢdᵢ are big and also, there is a common multiple between all dᵢ.