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