数学概率问题1. A hotel key card consists of for slots that can be

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数学概率问题
1. A hotel key card consists of for slots that can be programmed with either letters or digits. The six letters A,B,C,D,E,F and the ten digits 0 to 9 are used. The keys are programmed according to the following conditions: 1)the first slot must be a letter 2)the last slot must be a letter 3)the middle two slots can be either a digit or a letter 4)there are no repetitions of letters or digits how many different key cards are possible? answer：10920
2.How many 4 letters combinations can be formed from the letters A A A A B B B C D E F G H it there must be at least 2A’s or 2B’s in each combination? answer: 58
谢谢

"consists of for slots that " 这里不是for 是 four吧?
"it there must be at least 2A’s or 2B’s "这里是2As和2Bs吧?"it" 是 "if"吧?
如果是的话,解如下：
1.是组合问题：(6取1)*(5取1)*(4取2)*(10取2)=8100
2.还是组合问题：(4取2)*（3取2)*(9取2)=648
你的答案是正确的么?

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