Let $X$ be the discrete random variable representing the number $\left(x\right)$ appeared on the face of a biased die when it is rolled. The probability distribution of $X$ is

$X=x$	$1$	$2$	$3$	$4$	$5$	$6$
$P\left(X=x\right)$	$0.1$	$0.15$	$0.3$	$0.25$	$k$	$k$

Then variance of $X$ is