To find the class marks, the following relation is used.
xᵢ = (Upper Limit + Lower Limit)/2
Class size (h) for this data = 10. Tking 70 as assumed mean (a), dᵢ, uᵢ and fᵢuᵢ are calculated as follows.

From the table, we obtain
∑fᵢ = 35, ∑fᵢuᵢ = -2, a = 70 and h = 10
mean (X̄) = a + (∑fᵢuᵢ /∑fᵢ)h = 70 + (-2/35) × 10 = 70 – 4/7 = 70 – 0.57 = 69.43
Therefore, mean literacy rate is 69.43%