Let \( X \) be the discrete random variable representing the number (\( x \)) appeared on the face of a biased die when it is rolled. The probability distribution of \( X \) is as follows:
\[
X = x: \quad 1, \, 2, \, 3, \, 4, \, 5, \, 6
\]
\[
P(X = x): \quad 0.1, \, 0.15, \, 0.3, \, 0.25, \, k, \, k
\]
The variance of \( X \) is: