Question
A bowl contains 9 blue balls, 7 white, and 5 black
balls. If 2 balls are drawn in one go, then the probability of exactly one ball being blue is:Solution
In total, we have 9+7+5=21 balls. We have drawn 2 balls; we can draw them in 21C2 ways = 21! ÷ (19! ×2!) = 210 ways. In those two balls, we need one to be a blue ball, so we can include that in 9C1 ways = 9 ways. The remaining ball may be any color ball but not white, so we select other balls in 12C1 ways= 12ways. Probability= (9×12)/210 =18/35.
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