The original problem is somewhat trivial, and I have one last step to prove the mutually independence.
$P(Y=k)=1/n, k=0,1,...,n-1$
$n=p_1p_2…p_k$ and $p_1,p_2,...,p_k$ are co-prime integers.
Prove that $X_{p_i}=(Y\ mod\ p_i), i=1,...,k$ are mutually independent.
Any help would be appreciated.
(By the way, I have caculated the probability $P(X_{p_i}=k)=1/p_i, k=1,...,p_i-1$
But no idea of what to do next.)
Here is my solution to it, as a reference.