1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I'm fine with the second part (n = 6). But the first part is eluding me, I've been told it's quite simple. I feel like it's something to do with the fact that for each prime factor of C_n, p1, p2, p3, we have p-1|C_n-1 But don't really know how to use this.