math
posted by joe .
How do you do this question?
A shipment of 25 television sets contains three defective units. In how many ways can a vending company purchase four of these units and receive (a) all good units, (b) two good units, and (c) at least two good units.

ways to get 4 good units: 1 (good, good, good, good)
ways to get two good:
GGBB
GBBG
GBGB
BBGG
BGBG
BGGB
ways to get at least two good:
above ways, plus
GGGG
PLus
GGGB
GBGG
GGBG
BGGG 
BGGG
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