What is the probability that none are defective

In a shipment of 20 computers, 3 are defective. Three computers are randomly selected and tested. What is the probability that none are defective if the first and second ones are not replaced after being tested?
A. 4913/8000
B. 4913/6840
C. 34/57
D. 1/2000

In a shipment of 20 computers, 3 are defective. Three computers are randomly selected and tested. What is the probability that none are defective if the first and second ones are replaced after being tested?
A. 4913/6840
B. 1/2000
C. 4913/8000
D. 34/57

Without replacement:

(17 choose 3)(3 choose 0) / (20 choose 3)
  = [17!/(14!3!)] / [20!/(17!3!)]
  = (17*16*15/6) / (20*19*18/6)
  (17*16*15) / (20*19*18)
  = 34/57

With replacement:

(17/20)(17/20)(17/20) = 4913/8000

(The three selections are independent.)