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這是個概率問題:0不能做首位,所以千位只能有5,8,9三種選擇,百位則可能有除千位之外的三種選擇(這時0可以做百位,四個數除去千位用去的一個數,還剩兩個數位做選擇),十比特可以有兩種選擇(除去百位和千位的數位,還剩兩個數位),所以答案是3*3*2*1=18
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