among 18 computers, 12 are in working order and six are defective. How many sample of 4 are possible, selected without replacement and without regard to order, wherein all are defective?
Since we know beforehand that we're only choosing amongst the defective ones, we want to choose any four of the six defectives = C(6,4) = 15 possible different samples of defectives
We have 10 computers. 5 are taken, meaning being used, while the others are open. How many are open? If we bought 100 more computers, and the 5 that are being used were still used, how many computers would be open?
