Let $R = \mathbb Z/25\mathbb Z$ and $S = \{1,7,18,24\}$

1. How many elements of $R$ are non-zero-divisors? How many are invertible?
2. Now consider the subring $RS$. How many elements of $RS$ are non-zero, how many are zero-divisors, how many are invertible?

For (1) I think $R$ has 24 non-zero elements. Also $S$ is set of powers of $7\pmod {25}$.
Thanks for your help!