Question

Find the probability of randomly selecting 4 good quarts of milk in succession from a cooler containing 20 quarts of which 5 have spoiled

Answer 1

Need 4 good quarts out of 20, of which 5 have spoiled.

=>

Need 4 good quarts out of 15 good quarts out of 20.

Getting

P(G)=15/20
P(GG)=(15/20)(14/19)
P(GGG)=(15/20)(14/19)(13/18)
P(GGGG)=(15/20)(14/19)(13/18)(12/17)
=91/323

(fractions provide exact answers in probability problems.  Decimals are approximate most of the time).