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Urgent! ! ! Elementary school math problems about permutation and combination
There are 4+3+2=9 kinds of questions1* * (you can take science books, 4 kinds of books, comics, 3 kinds of books, literature books and 2 kinds of addition to complete this task).

Question 2 * * * has 4*3*2=24 kinds.

When you take the first science and technology book, you can choose three comic books and two literature books. When I am a sophomore, I can choose three comic books and two literary books. When taking the third science and technology book, you can choose three comic books and two literature books. When you take the fourth science and technology book, you can choose three comic books and two literature books, so you use multiplication. )