A bag contains 5 red balls and 4 blue balls. In how many ways can 3 balls be selected so that at least 2 are red?
A40
B50CORRECT
C70
D80
GCEFurther MathematicsPermutations and Combinations2020
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Why the answer is B, and why the others tempt you.
At least 2 red means exactly 2 red + 1 blue, or exactly 3 red. C(5,2)×C(4,1) + C(5,3)×C(4,0) = 10×4 + 10×1 = 40 + 10 = 50. Option C (70) equals C(9,3), the total selections without restriction.
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