A company manufacturing bolts has 3 machines . machine A is older and produces 30% of the product.machince A and C produces half of the remaining product of the bolt produced by machine A , 5% or defective .the defect rate of the machine B and C is respectively 3% and 2%. one bolt produced at this factory is randomly choosen and is found to be defective . what is the probability that it was produced by machine B ?