How do you do this question?

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.

Answer 1

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