Help with statistic - probability rules?

A manufacturing firm orders computer chips from three different companies: 10% from Company A; 20% from B; and 70% from Company C. Some of the computer chips that are ordered are defective: 4% from Company A; 2% of chips from B; and 0.5% of chips from Company C are defective. A worker at the manufacturing firm discovers that a randomly selected computer chip is defective. What is the probability that the computer chip came from Company B?