Math

A box contains 8 white balls, 7 black and 4 red. How many red balls must be added to the box so that the probability is 3/4? How many black balls need to be added so that it is 1/4?
Thanks I have a mental block with this.

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  1. probability of what?

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  2. Sorry...
    so that the probability of drawing a red ball is 3/4

    so that the probability of drawing a white ball is 1/4

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