请教：Susan的数学问题...

zoewhq · 发表于 2010-9-6 08:36:38

suan has selected exactly three crayons,each a different color,from a crayon box.how many crayon colors were represented in the box before her selection?
a) exactly four distinct three-crayon color combinations were represented in the box before susan selected three crayons
b)just before susan's selection,the box contained exactly five crayons.

答案给的C，但是我不明白

孤独的长跑者 · 发表于 2010-9-6 09:05:31

为什么不是A？C(4,3)=4足够了。

zoewhq · 发表于 2010-9-6 09:14:47

我也是这么认为的，纠结了一个多小时...
而且我认为条件2起不到任何作用，太宽泛了

zoewhq · 发表于 2010-9-6 09:34:30

这个是这道题给的解释：
statement1alone is sufficient to answer the question.for example,letting letters represent colors,the box could include four crayons and four different colors-a,b,c, and d-offering four distinct three-crayon combinations:{a b c}.{abd}.{acb}.and {bcd}. as an alternative, the box could include five crayons but only three different colors-{a a a b c}-offering否认distinctthree-crayoncombinations：{abc}.{aaa}.{aab}.and {aac}.statement 2 alone is also insufficient to answer the question.Prior to susan's selection, the box could have represented anywhere from three to five colors.again,using letters to represent colors,possible combinations include {aaabc},{aabcd}.and {abcde},to list just a few.together ,statement 1 and 2 suffice to answer the question.exactlythree colors must be represented in a box of fivr crayonsoffering exactly four three-crayon combinations.again,using letters to represent colors, the configuration must be {a a a b d}.all other five- crayon configuration of three or more colors offer too many three-crayon color combinations.The closest is {aabcc},which offers five combinations:{aab}.{aac}.{abc}.{abb} and{bbc}

请教：Susan的数学问题...

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