Yeah, I agree - your counselor shouldn't suggest a class you literally shouldn't be able to take
The pyhtagorean states that to find c, use the eqaution,

.
Substitute 1 for a and 20 for b;
c^2=1+400
c^2=401
c=
which is about 20.02
Hope this helps.
You can get a app to help you with this