[理工] 離散數學 排列機率

[nightnawk (西西)]

What's the minimum number of people such that the
 probability that two of them were born on the same
 day of the week at least 80?

它答案寫五

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→ kiki86151:題目問n人在一個禮拜同天出生機率 可以先想n人在不同天
→ kiki86151:出生機率 最後1在扣掉>=80% 求n
→ kiki86151:1-[7*6*...*(7-n+1) / 7^n] >=80%  n=5
→ kiki86151:如果不懂 可以先試著想2人在不同一天出生機率比較好理解
→ kiki86151:7*6(A先選 剩下在給B選) / 7*7 (A和B的組合也就是7^2)
→ kiki86151:同一天就是1-( 7*6 / 7^2) 最後可以用數歸法求到n人的
→ nightnawk:謝謝指導 只是這是要慢慢帶出答案嗎??