4. Do the following:
(a) Suppose that $X_1$, $X_2$ are discrete iid random variables with probability mass function
$$p_{X_i}(x) = \begin{cases}
0.4 & x = 0 \\
0.3 & x = 1 \\
0.3 & x = 2
\end{cases}$$
i. (6 pts) Compute the joint probability mass function $p_{X_1,X_2}(x_1, x_2)$.
ii. (6 pts) Let $T = X_1 + X_2$. Compute the probability mass function of $T$.
iii. (8 pts) Compute $E(X_1)$, $V(X_1)$ and use it to compute. $V(T)$.