2. 外语
  3. Hal has 4 girl friends and 5 boy friends. In how many different ways can Hal invite 2 girls and 2 boys to his birthday party?

Hal has 4 girl friends and 5 boy friends. In how many different ways can Hal invite 2 girls and 2 boys to his birthday party?

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问题 Hal has 4 girl friends and 5 boy friends. In how many different ways can Hal invite 2 girls and 2 boys to his birthday party?
选项 A、54
B、60
C、72
D、120
E、240
答案B
解析 第一步选择女孩，从4个人里面选择24个人C42；第二步选择男孩，从5个人里面选择2个人C52。
根据乘法原理可以得出C42×C52=60。所以答案为B选项。
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