Ah, that one. First you find the assumed mean 'A' and step deviation 'd_i' for each class, then use the formula for mean: x̄ = A + h(Σf_id_i / Σf_i). Standard deviation σ comes from √[h²(Σf_id_i²/Σf_i - (Σf_id_i/Σf_i)²)]. Honestly, I just keep redoing the sums from our last chapter’s problems—otherwise, with the roof leaking in the store-room, my notebook gets damp and all the khann blurs.
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