The sample space is:
| BBBB | ||||||
| GBBB | BGBB | BBGB | BBBG | |||
| GGBB | GBGB | GBBG | BGGB | BGBG | BBGG | |
| GGGB | GGBG | GBGG | BGGG | |||
| GGGG |
(a). P(exactly 3 boys) = 4/16 = 1/4
(b). P(exactly 2 girls) = 6/16 = 3/8
(c). P(at least 2 boys) = P(2 or 3 boys) = P(2 boys) + P(3 boys) = 6/16 + 4/16 = 10/16 = 5/8
(d). P(at most 2 girls) = P(0, 1 or 2 girls) = P(0 girls) + P(1 girl) + P(2 girls) = 1/16 + 4/16 + 6/16 = 11/16