Two methods, A and B, are available for teaching a skill. The failure rate for method A is 30% and 10% for B. Method is more expensive, however, and hence used only 20% of the time (A 80%). A worker is taught the skill by one of two methods but fails to learn the skill. What is the probability he was taught by method A?

I am looking for P(A| failed)?

Also, I am not sure hoe set this up if that is correct.