Probability of mutually independent random variables I have been doing an exam review and this is one of the problems I could not solve.

Let $X_1, X_2, ..., X_n$ be a sequence of mutually independent random variables.
For each $i$ with $1 \leq i \leq n$,


*

*the variable $X$, is equal to either $0$ or $n+1$

*$E(X_i) = 1$

Determine $Pr(X_1+ X_2 + ... + X_n  \leq n)$


Hint: (i) Find $\Pr(X_i=n+1)$; (ii) Our sum is $\le n$ if and only if all the $X_i$ are equal to $0$. 
