If the variance of the terms in an increasing A.P. b?,b?,b?,...,b?? is 90, then the common difference of this A.P. is 

The variance of 20 observations is 5. If each observation is multiplied by 2 then the new variance of the resulting observations, is:
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Let xᵢ (1 ≤ i ≤ 10) be ten observations of a random variable x. If Σ(from i=1 to 10)(xᵢ – p) = 3 and Σ(from i=1 to 10)(xᵢ – p)² = 9 where 0 ≠ p ∈ R, then the standard deviation of these observations is: