There are 4 white balls and n yellow balls in an opaque cloth bag. They are all the same except for different colors. If a ball is drawn out at random, the probability that it is a white ball is (1/5), what is n ．
在一个不透明的布袋中装有4个白球，n个黄球，它们除颜色不同外其余均相同，若从中随即摸出一个球，它是白球的可能性是(1/5)，则n多少．
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