Consider the following probability mass function (p.m.f) of a random variable $X:$
$$p(x,q) = \begin{cases} q & \text{ if } X=0 \\ 1 – q & \text{ if } X=1 \\ 0 & \text{otherwise} \end{cases}$$
If $q=0.4$, the variance of $X$ is _________