Primitive roots of unity

A complex number $\theta$ is said to be a primitive $n^{th}$ root of unity if $\theta^n=1$, but $\theta^m \neq 1$ for any positive integer $m$ lesser than $n$. It is easy to see that $n^{th}$ roots of unity form a cyclic group. Hence by the definition of primitive root, we can see the primitive roots are nothing but the generators of the cyclic group of roots of unity.
The following are some questions asked in TRB/CSIR questions.
1. What are the primitive $20^{th}$ roots of unity?
2. Find the number of primitive $20^{th}$ roots of unity?
(Few more questions will be updated soon.)