A jar contains 20 marbles:4 red,6 white and 10 blue.If you r

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A jar contains 20 marbles:4 red,6 white and 10 blue.If you remove one at a time,randomly,what is the minimum number that must be removed to be certain tat you have at least 2 marbles of each colors?
A) 6 B) 10 C) 12 D) 16 E) 18
想问下答案是怎么算出来的,如果加减呢?我尝试用概率可是不知道怎么理解,

中文意思是 4 红,6白,10蓝,每次随机拿一个球,问最少需要拿多少个才能确定里边含有2红、2白、2蓝.
我的思路是这样,考虑极端恶劣情况：
前10个,均为蓝.已拿10个,完成2个蓝.
再拿2个,均为白.已拿12个,完成2蓝,2白.
再拿2个,还是白.已拿14个,完成2蓝,2白.
再拿2个,还是白.已拿16个,完成2蓝,2白.
再拿2个,嗯 ,终于是红了.已拿18个,完成2蓝、2白、2红.
所以答案是E

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