If the average of a group of sample data 2, 3, 7, 8, a is 5, then the variance S2 of the group of data is 0=______ .
∵ the average of data 2, 3, 7, 8, a is 5, ∵ 2 + 3 + 7 + 8 + a = 25, the solution is a = 5, ∵ variance S2 = 15 [(2-5) 2 + (3-5) 2 + (7-5) 2 + (8-5) 2 + (5-5) 2] = 265
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