Let A denote a headache and B denote a fever.

Therefore, we have:

Bayes Rule states:

Evaluating, we get:

Now, use Bayes Rule again for our question that asks for a headache given a fever. That is:

Evaluating, we get:

If you ever have a complement problem using Bayes Rule, you can use my calculator. It won't solve this particular problem, but I'll build that in the future. It solves certain other problems:

Bayes Rule