Distinguish a transformed observation from two independent observations

The discrete random variable $X$ has distribution

$x$	0	1	2	4	8
$P(X=x)$	$p$	$1/2$	$1/4$	$1/8$	$1/16$

(i) Evaluate $p$. (ii) Find $E(X)$ and $\operatorname{Var}(X)$. (iii) Write down $E(2X-3)$ and $\operatorname{Var}(2X-3)$. (iv) For two independent observations $X_1,X_2$ of $X$, find $E(X_1+X_2)$ and, using $p$ from (i), find $P(X_1+X_2=2)$.