permutation and combination
2012-01-19 2:23 am
in how many was can 4 letters be seletced from the word "document" if the letters"o" and "e" cannot be seletced at the same time.ans:55please give me a step and reason
回答 (1)
2012-01-19 2:34 am
✔ 最佳答案
Required ways= All ways - o and e are seletced at the same time
= 8C4 - o,e ways * other any 2 letters among the remains ways
= 8C4 - 2C2 * (8-2)C2
= 8C4 - 1 * 6C2
= 70 - 15
= 55 ways
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