推 iuz : 非常感謝! 05/02 19:48
※ 引述《iuz (iuz)》之銘言：
: 請問以下這道題目:
: 從0123456789這十個數字中任取三個相異三數作為三位數，
: 則此三位數是偶數且大於520有幾個?
: Ans:174
: Thanks!
(1) 百位是 6,8: 個位有 4 種, 再選十位有 8 種, 計 2*4*8 = 64
(2) 百位是 7,9: 個位有 5 種, 再選十位有 8 種, 計 2*5*8 = 80
(3) 百位是 5: 先不管大於 520 這條件, 則和 (2) 一樣有 5*8 = 40 個
然後再扣掉 ≦520 的
共有 520, 518, 516, 514, 512, 510, 508, 506, 504, 502 十個
故這裡計 30 個
全部總計 64+80+30 = 174 個
--
'You've sort of made up for it tonight,' said Harry. 'Getting the
sword. Finishing the Horcrux. Saving my life.'
'That makes me sound a lot cooler then I was,' Ron mumbled.
'Stuff like that always sounds cooler then it really was,' said
Harry. 'I've been trying to tell you that for years.'
-- Harry Potter and the Deathly Hollows, P.308
--
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