Two methods, a and b, are available for teaching a certain industrial skill. the failure rate is 20 percent for method a and 10 percent for method
b. method b is more expensive, however, and hence is used only 30 percent of the time. (method a is used the other 70 percent of the time). a worker is taught the skill by one of the two methods, but he fails to learn it correctly. what is the probability that he was taught by method a?