求助一道PREP题···完全不知道他在讲什么哇··

devilmona

If 500 is the multiple of 100 that is closest to x and 400 is the multiple of 100 that is closest to y. Which multiple of 100 is closest to x+y?
(1). x<500
(2). y<400

啥意思啊？求牛牛指教···

devilmona

谢谢楼上···大牛一生平安···

Jas敏

题目的意思是500是和x相差最小的整百数
即450<x<550
400是与y相差最小的整百数
350<y<450
那么x+y的话就是 800<x+y<1000
(1)when x<500, then 450<x<500, 800<x+y<950
   与x+y相差最小的整百数可以是800 也可以是900 ---not sufficient
   （也可举反例证明 当x=460, y=360时，x+y=820, 答案是800；
   当x=490， y=440时， x+y=930， 答案是900）
剩下的用同样的思路

milon123

If 500 is the multiple of 100 that is closest to x and 400 is the multiple of 100 that is closest to y. Which multiple of 100 is closest to x+y?
(1). x<500
(2). y<400

SOLUTION.
From the first part of the sentence, get 450<x<550 and from the second part, 350<y<450.
(1) If x<500, then 450<x<500, i.e. 800<x+y<950, the mutiple of 100 that is closeset to x+y will be 800 or 900.
(2) If y<400, then 350<y<400, i.e. 800<x+y<950, the mutiple of 100 that is closeset to x+y will be 800 or 900 as well.
When (1) and (2) are both held,
then 450<x<500 and 350<y<400, i.e. 800<x+y<900, the mutiple of 100 that is closeset to x+y will be 800 or 900 as well.

So, (1) combined with (2) is not sufficient.

The key is E. Is it right?

Jas敏

答案是E吗？
Both together are not sufficient

devilmona

顶啊···求指教啊···

flora_yj

同问~这道题我也做错了，实在是不明白在说什么==