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For grouped data the individual values are unknown, so each class is represented by its midpoint. The mean is then Σfx / Σf, which is exact only if the values are evenly spread within each class.
Grouped mean
x̄ = Σ(f·x) / Σf, using class midpoints for x
Σf = 40 and Σfx = 20 + 120 + 375 + 315 + 180 = 1010, so the mean is 25.25.
Because using the midpoint assumes symmetry within each class; skew inside a class biases the result.