Suppose that two defective refrigerators have been included in a shipment of six refrigerators. The buyer begins to test the six refrigerators one at a time.

a) What is the probability that the last defective refrigerator is found on the fourth test?

b) What is the probability that no more that four refrigerators need to be tested to locate both of the defective refrigerators?

c) When given that exactly one of the two defective refrigerators has been located in the first test, what is the probability that the remaining defective refrigerator is found in the third or fourth test?

Don't know where to begin here =(...