Assume an office has two telephones (A and B) with separate lines and different telephone numbers. The probability that telephone A rings when the office is closed is .30. The probability that telephone B rings when the office is closed is .20

What is the probability that A rings given that B rings when the office is closed?

I am assuming that A rings when the office is open.

If the events are independent, the probability of both/all events occurring is determined by multiplying the probabilities of the individual events.

(1-.30)(.20) = ?