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A data consists of n observations : x 1 , x 2 , ....., x n . If ∑ = + n 1 i 2 i ) 1 x ( = 9n and ∑ = n 1 i 2 i ) 1 – x ( = 5n, then the standard deviation of this data is :

1. 2
2. 5
3. 5
4. 7

Correct answer: 5

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