Question

There are 10 rolls of film in a box and 3 are defective. two rolls are selected without replacement. what is the probability of selecting a defective roll followed by another defective roll?

Answer 1

P(picking one defective) = 3/10
P(picking a 2nd defective) = 2/9


P(1 and 2  defective) = 3/10 x 2/9  = 6/90 = 0.066

Second method using combination:
 ³C₂ / ¹⁰C₂ = 1/15 = 0.066