Re: minor mistake -- SFXC forum

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Subject: Re: minor mistake

Author:
Bobby仔

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Date Posted: 11:23:11 05/03/03 Sat
In reply to: SomeOne 's message, "minor mistake" on 06:02:16 05/03/03 Sat

>>>MC題:One out of ten tins of milk powder contains a
>>>prize slip. What is the least number of tin that
>>>should be bought so that the probability of getting
>at
>>>least one prize slip exceeds 90%?
>>> 不才想來想去以為答案是10,但model
>>>answer是22,點解?望有識之同學師兄師長指點,萬分多謝.
>>先計取得N罐都無prize既prop.
>>即係 (0.9)^n
>>要令到個抽到既prop是90%
>>咁 (0.9)^n就要細過0.1
>>所以 0.9^n < 10
>>用cal計一計
>>n > 21.xxxxxxx
>>即係要n = 22
>
>應該係 0.9^n < 0.1
>所以 n > 21.85
>因為 n 係 integer, 所以 n = 22.
謝!!
漏打 "%"

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