This is a data sufficiency problem from 《30 days to the GMAT CAT 2nd edition》sets
Susan 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?
(1) Exactly four distinct three-crayon color combinations were represented in the box.
(2) Just before Susan's selection, the box contained exactly five crayons.
The answer is C.
I have read the explaination several times,but I still can't understand it.Can anyone give me a clear method or just a translation of the question itself?
Thank you very much!
S要从盒子中挑出三种不同颜色的粉笔,在她挑之前,盒子里的粉笔一共有多少种颜色?
(1)盒子里有四种不同的三色组合;
(2)S挑之前,盒子里有5支粉笔;
我的意见:本来我是觉得(1)单独就足够了,因为C(n,3)=4=n(n-1)(n-2)/(3*2*1),则n=4;管他有几支粉笔(可能会有相同颜色的粉笔存在...),反正颜色是4种就okay啦...
可是他的答案是C,我又想了想...可能是因为条件(1)并没有说明是挑之前或者挑之后的情况吧...寒!(ETS有这么BT吗?),所以加上2的条件,可以知道,盒子里有4种颜色,至少要有4支粉笔吧...挑之前只有5支,所以(1)所说的是在挑之前的情况...
所以两个条件合起来,可以推出,在S挑之前,有5支粉笔,四种颜色(有两支同色)的粉笔在盒子里...
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