Hyper geometric distribution: a box consist of 7 black and some white balls....

a box consist of 7 black and some white balls. the probability of selecting 2 white balls is 1/15 ,find no of white balls.

Bilalali97 said:

a box consist of 7 black and some white balls. the probability of selecting 2 white balls is 1/15 ,find no of white balls.

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Let the number of white balls be x. Then the total number of balls is x+ 7. The probability the first ball chosen at random is white is \(\displaystyle \frac{x}{x+ 7}\). Given that, there are x+ 6 balls left, x- 1 of them white. The probability the second ball chosen is also white is \(\displaystyle \frac{x-1}{x+ 6}\).

The probability both balls are white is \(\displaystyle \frac{x}{x+ 7}\frac{x- 1}{x+ 6}\).