Hint: let $s$ be one of the faces of the dodecahedron. The _orbit_ of $s$ is the set of faces to which $s$ can be mapped under the action of the group; can you figure out how many elements $s$ can be mapped to? (Don't forget to count $s$ itself) Likewise, the _stabilizer_ is the set of symmetries that leave $s$ 'in place'; can you figure out how many of those there are?
As another check on this, you could choose to let $s$ be one of the _vertices_ of the dodecahedron rather than its faces; you should find the same result by multiplying the size of the orbit by the size of the stabilizer.