The variance of the first $n$ even natural numbers is:
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Since the first $n$ natural numbers have a variance of $\frac{n^2-1}{12}$, scaling them by 2 (even numbers) scales the variance by $2^2 = 4$. Thus, $4 \times \frac{n^2-1}{12} = \frac{n^2-1}{3}$.