OG PS # 218 求简易做法

SimingGMAT · 发表于 2012-12-18 09:31:27

List T consist of 30 positive decimals, none of which is an integer, and the sum of the 30 decimals is S. The estimated sum of the 30 decimals, E, is defined as follows. Each decimal in T whose tenths digit is even is rounded up to the nearest integer, and each decimal in T whose tenths digits is odd is rounded down to the nearest integer. If 1/3 of the decimals in T have a tenths digit that is even, which of the following is a possible value of E - S ?

I. -16
II. 6
III. 10

A. I only
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III

选B

Annapp · 发表于 2012-12-18 19:25:42

我不知道OG的方法是什么，这样想不知道算不算简便方法

就是你假设所有的十分位是偶数的decimals的十分位都是一样的，所有的十分位是奇数的decimals的十分位也都是一样的。
用【偶】、【奇】分别表示估计值跟原值相差的绝对值。

那么E-S= 10×偶－20×奇
考虑极端情况：偶最小是0.2，奇最小是0.1
只要偶=0.2，只要 -16≤E-S≤0 就OK
只要奇=0.1，只要 0≤E-S≤6 就OK

OG PS # 218 求简易做法

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