3.76 A manufacturer produces boxes of candy, each containing 10 pieces. Two machines are used for this purpose. After a large batch has been produced, it is discovered that one of the machines, which produces 40% of the total output, has a fault that has led to the introduction of an impurity into 10% of the pieces of candy it makes. The other machine produced no defective pieces. From a single box of candy, one piece is selected at random and tested. If that piece contains no impurity, what is the probability that the faulty machine produced the box from which it came?