Let the point $(u, v)$ be chosen randomly in the $[0, 1] \times [0, 1]$ square. Let $X$ be the random variable that associates to point $(u, v)$ to the number $(u + v)$. Obtain the distribution function of $X$.
Thoughts:
$0 \leq u \leq 1$, and $0 \leq v \leq 1$. Therefore $0 \leq u + v \leq 2$. But $X = u + v$, so we have $0 \leq X \leq 2$. I'm not sure how to take it from here, but I think that
$$ F_X = \frac{x}{2}, ~~ 0 \leq x \leq 2 $$