The variance of first 50 even natural numbers is

We know that $\mathrm{Var}\left(aX\right)=a^2\mathrm{Var}\left(X\right)$

$\therefore \mathrm{Var}(2,4,6,\dots ,100)=4\mathrm{Var}(1,2,3,\dots ,50)$

$=4\left(\frac{{50}^2-1}{12}\right)\left[\because \mathrm{Var}(1,2,\dots ,n)=\frac{n^2-1}{12}\right]$

= 833