题干

老师从写有1~13的13张卡片中,抽出9张,分别贴在9位同学的额头上,大家能看到其他8人的数但看不到自己的数.(9位同学都诚实而且聪明,且卡片6、9不能颠倒)老师问:“现在知道自己的数的约数个数的同学请举手.”有两人举手,手放下之后,有三个人有如下的对话.

甲:“我知道我是多少了.”

乙:“虽然我不知道我的数是多少,但我已经知道自己的奇偶性了.”

丙:“我的数比乙的小2,比甲的大1.”

那么,没有被抽出的四张牌上数的和是____ .

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