Three factories produce light bulbs to supply the market. Factory a produces 20%, 50% of the tools are produced in factories b and 30% in factory c. 2% of the bulbs produced in factory a, 1% of the bulbs produced in factory b and 3% 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 b?.