A box contains 5 blue, 6 yellow and 4 red balls. The number of ways, of drawing 8 balls containing at least two balls of each colour, is :
- 4100
- 4140
- 4230
- 4290
Answer
(1) 4100
Solution
Let b, y, r be the numbers of blue, yellow and red balls drawn: b+y+r=8, b, y, r 2 (b 5, y 6, r 4).
after 2 of each colour, 2 more balls remain: (b, y, r)=(4,2,2), (2,4,2), (2,2,4), (3,3,2), (3,2,3), (2,3,3)
(4,2,2): 5C4 6C2 4C2=5 15 6=450
(2,4,2): 5C2 6C4 4C2=10 15 6=900
(2,2,4): 5C2 6C2 4C4=10 15 1=150
(3,3,2): 5C3 6C3 4C2=10 20 6=1200
(3,2,3): 5C3 6C2 4C3=10 15 4=600
(2,3,3): 5C2 6C3 4C3=10 20 4=800
Number of ways =450+900+150+1200+600+800=4100
Answer: (1)