求教，具体分析过程

hucongzi722 · 发表于 2011-2-24 23:51:44

1. If P is a set of integers and 3 is in P, is every positive multiple of 3 in P ?
(1) If x is in P, then x+3 is in P.
(2) If x is in P, then X-3 is in P.

2. Jerome wrote each of the integers 1 through 20, inclusive, on a separate index card. He placed the cards in a box, them drew cards one at a time randomly from the box, without returning the cards he had already drawn to the box. In order to ensure that the sum of all the cards he drew was even, how many cards did Jerome have to draw?
A.19 B.12 C.11 D.10 E.3

3. A square countertop has a square tile inlay in the center, leaving an untiled strip of uniform width around the tile. If the ratio of the tiled area to the untiled area is 25 to 39, which of the following could be the width, in inches, of the strip?
a.1 b.3 c. 1 and 3 d.1 and 4 e. 1, 3 and 4

4. Exactly three years before the year in which Anna was born, the year was 1980-x. In terms of x, on Anna's twentieth birthday, the year will be?
a.1977+x b.1997+x c. 2003-x d.2003+x e.2006+x

answers: 1.A 2.B 3.E 4.C

kazemi · 发表于 2011-2-25 00:08:02

http://www.beatthegmat.com/integers-t11142.html

kazemi · 发表于 2011-2-25 00:11:22

http://www.beatthegmat.com/combinatorics-t55282.html

hucongzi722 · 发表于 2011-2-25 00:16:32

thank you for these two questions!

kazemi · 发表于 2011-2-25 00:17:33

http://www.beatthegmat.com/square-countertop-t21427.html

kazemi · 发表于 2011-2-25 00:20:37

4.

A-3=1980-x

A=1983-x

A+20=1983-x+20=2003-x

hucongzi722 · 发表于 2011-2-25 00:37:01

why 4 ?

I think: anna 是1980-X-3 出生的？这一年是1980-X, 而她是在这一年的三年前出生？

hucongzi722 · 发表于 2011-2-25 00:39:44

题目没理解好。
I got: anna 是1980-X+3 出生的.

求教，具体分析过程

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