Your question is Bayes Rule and Model Design. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Explain how you would use Bayes' rule to compute P(A|B) from P(B|A), P(A), and P(B), and describe how you would design a probabilistic model for that problem.