求助PREP一道数学题目

jennyruxin · 发表于 2009-11-25 12:12:37

10. 719-!-item-!-187;#058&000515

Of the 800 students at a certain college, 250 students live on campus and are more than 20 years old. How many of the

800 students live on campus and are 20 years old or less?

(1) 640 students at the college are more than 20 years old.

(2) 60 students at the college are 20 years old or less and live off campus.

请问怎么做啊？哪位大侠教教，我是数学白痴

lovesasa0820 · 发表于 2009-11-25 12:25:02

是不是是C啊？800人中有4类人，分别是住校大于20岁，住校小于等于20岁，住校外大于20岁，住校外小于等于20岁
四类加起来800
只要知道其中3个可以求另外一个了

求助PREP一道数学题目

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