The mean and variance of n observations ${x}{1},{x}{2},{x}{3},..............{x}{n}$ are $5$ and $0$ respectively. If $\displaystyle\sum { i=1 }^{ n }{ { x }{ i }^{ 2 }=400 } $, then n is
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The mean and variance of n observations ${x}{1},{x}{2},{x}{3},..............{x}{n}$ are $5$ and $0$ respectively. If $\displaystyle\sum { i=1 }^{ n }{ { x }{ i }^{ 2 }=400 } $, then n is