An urn contains 15 white and 21 black balls. Balls are drawn one by one, without replacement, until 6 white balls are drawn. Find an expression for the probability that the total number of balls drawn is x.
The probability(p) of drawing a white ball
For the total number of balls drawn to be x, there should be 5 white balls among the first x-1 balls that are drawn.
Number of ways of selecting 5 white balls among x-1 balls is
No. of black balls among the first x-1 balls
The total probability of selecting 5 white balls and x-6 black balls among the first x-1 balls and then selecting a white ball in draw
Hence, the expression for the probability that the total number of balls drawn is x
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