Number of Distinct ideals of $Z_{60}?$ Tried to count all prime numbers between $0$ to $60$ and adding $(0)$ and (R) to it.
that is total $19$ , but I saw that answer is $18$.
So, Please explain.

$\tau(60)=12$ so there are $12$ ideals
Note: The ideals in $Z_n$ are precisely the sets of the form $<d>$ where $d$ divides $n$, so number of ideals are same as the number of divisors of $n$
