4. A factory has three workshops W1, W2 and W3 making the same spark plugs, respec- tively in proportions 20%, 30% and 50% of the total production. The probability of a defective spark plug at W1, W2 and W3 is respectively 0.02, 0.03 and 0.05. (a) If we randomly select a spark plug from the total production, what is the prob- ability that it is defective? (b) Given that a randomly selected spark plug is defective, what is the probability that it comes from workshop W1