To find out the concentration of SO₂ in the air (in parts per million, i.e., ppm), the data was collected for 30 localities in a certain city and is presented below:

To find the class marks for each interval, the following relation is used.
xᵢ = (Upper Limit + Lower Limit)/2
Class size of this data = 0.04. Taking 0.14 as assumed mean (a), dᵢ, uᵢ, fᵢuᵢ are calculated as follows.

From the table, we obtain
∑fᵢ = 30, ∑fᵢuᵢ = -31, a = 0.14 and h = 0.04
mean(X̄) = a + (∑fᵢuᵢ / ∑fᵢ)h = 0.14 + (-31/30) × (0.04) = 0.14 – 0.04133 = 0.09867 = 0.099
Therefore, mean concentration of SO₂ in the air is 0.099 ppm.

See this Video explanation for better understanding😀✌