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Find the number of blue balls

Posted January 31st, 2008 by alpha
in

A bag contains 5 red balls and some blue balls . If the probability of drawing a blue ball is double of a red ball.Find the number of blue ball in the bag.

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answer

On January 31st, 2008 Isoscel says:

b = number of blue balls

$$Probability=\frac{favorable \: cases}{possible \:cases}$$
$$Prob\: blue\: ball=\frac{b}{b+5}=2(Prob\: red\: ball)=2\frac{5}{b+5}$$

or

$$b=2*5=10\:blue\: balls$$

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