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).