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89. In how many ways can 30 different books be arranged into three stacks so that each stack contains 10 books?

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89. A n s w e r. $\frac{1}{6} C_{30}^{10} \cdot C_{20}^{10}=30!:(10!\cdot 10!\cdot 10!\cdot 3!)$.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.