Question
A box has ‘x’ blue balls and 9 yellow balls. If the
probability of selecting a blue ball is 3/7, then find the probability of drawing two balls of different colours.Solution
ATQ, Total balls = x + 9 So, x/(x + 9) = 3/7 Or, 7x = 3x + 27 So, x = 27/4 = 6.75 (not possible as balls are integers, so x = 7 approx) Thus, x = 7 Desired Probability = (7C1 × 9C1)/(16C2) = (2 × 7 × 9)/(16 × 15) = 21/40
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