Probability of taking out white balls = (probability of taking out white balls from the first box * probability of taking out the first box)+(probability of taking out white balls from the second box * probability of taking out the second box)+(probability of taking out white balls from the third box * probability of taking out the third box) = (1/5) × (1)

Under the condition of getting a white ball, the probability from the first box = the probability of getting a white ball from the first box/the total probability of getting a white ball = (1/5) * (1/3)/(53/120) = 8/53, which is about/kloc.

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It is a key step in the process of solving probability mathematics problems, and solving probability mathematics problems is a good habit. There are too many examples of mistakes caused by incorrect understanding of the subject.

In order to determine the value of a random variable through "the value of a random variable", it should be accurate, especially when the random variable can be 0; The probability distribution of "event type correlation" is usually caused by equal probability events, random events, mutually exclusive events, relative events, independent events and independent repeated events. Before calculating the corresponding probability, it is necessary to determine the type of event.