Probability- general multiplication rule to find indicated probability

Question

a sample of 4 different calculators is randomly selected from a group containing 48 that are defective and 20 that have no defects. What is the probability that all four of the calculators selected are defectiv?

Answer 1

Sample size = 68, of which 48 are defective.

 

# ways to choose 4 defectives is 48C4 = 194580.

# ways to choose any 4 is 68C4 = 814385.

 

P(all 4 defective( = ratio of the above two numbers = 0.23893