I'm not able to get the exact solutions of `f`, what is unevaluated `RootOf` object. I doubt this is the expected behaviour.

```
In [31]: solve(f, quintics=True)
Out[31]: 
[RootOf(x**5 - 15*x**3 - 5*x**2 + 10*x + 20, 0),
 RootOf(x**5 - 15*x**3 - 5*x**2 + 10*x + 20, 1),
 RootOf(x**5 - 15*x**3 - 5*x**2 + 10*x + 20, 2),
 RootOf(x**5 - 15*x**3 - 5*x**2 + 10*x + 20, 3),
 RootOf(x**5 - 15*x**3 - 5*x**2 + 10*x + 20, 4)]
```