Three factories produce light bulbs to supply the market. Factory A produces 40%, 20% of the tools are produced in factory B and 40% in factory C. 5% of the bulbs produced in factory A, 3% of the bulbs produced in factory B and 4% of the bulbs produced in factory C are defective. A bulb is selected at random in the market and found to be defective. What is the probability that this bulb was produced by factory A?