Question
A sack contains 'r' silver balls and 12 gold balls. If
the probability of getting a silver ball is 1/4, then find the probability of getting different coloured balls when two balls are drawn randomly from the sack.Solution
Total number of balls in the sack = r + 12
So, r/(r + 12) = 1/4
Or, 4r = r + 12
Or, 3r = 12
So, r = 4
Total balls = 4 + 12 = 16
Desired Probability = (4C1 × 12C1)/16C2 = (4 × 12)/(16 × 15/2) = 48/120 = 2/5
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