Thirty women were examined in a hospital by a doctor and the number of heartbeats per minute were recorded and summarised as follows. Find the mean heartbeats per minute for these women, choosing a suitable method.

To find the class mark of each interval (x), the following relation is used.
xᵢ = (Upper limit + Lower limit)/2
Class size, h, of this data = 3
Taking 75.5 as assumed mean (a), dᵢ, uᵢ, fᵢuᵢ are calculated as follows.

From the table, we obtain
∑fᵢ = 30, ∑fᵢuᵢ = 4, a = 75.5 and h = 3
mean(X̄) = a + (∑fᵢuᵢ / ∑fᵢ)h = 75.5 + (4/30) × 30 = 75.5 + 4/10 = 75.5 + 0.4 = 75.9
Therefore, mean heartbeat per minute for those women are 75.9 beats per minutes.

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