有三堆棋子，每堆棋子数一样多。都只有黑白两种颜色，第一堆中的黑子，和第二堆中的白子一样多。第三堆中的黑子占全部黑子的2/5，求三堆棋子中，白子占全部棋子的几分之几？

（这个题目有点绕人，如果一时想不明白，还是先画线段图表示吧！）

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