Cup A contains 3mml of pure alcohol, and cup B contains 2mml of pure water. Pour a ml from cup A into cup B. After stirring well, pour a ml from cup B into cup A, then At this time, the pure water mixed in cup A is more than the pure alcohol mixed in cup B by how many milliliters.
A杯中盛有3m毫升纯酒精，B杯中盛有2m毫升纯净水，从A杯倒出a毫升到B杯里，搅匀后，又从B杯倒出a毫升到A杯里，则这时A杯中混入的纯净水比B杯中混入的纯酒精多多少毫升．
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