[求助]GWD9-3/9-7/9-19

下面几题都不会做，诚心请教啊！

GWD 9

19.Is the integer n odd?

(1)   n is divisible by 3.

(2)   2n is divisible by twice as many positive integers as n.   

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONsufficient.

D. EACH statement ALONE is sufficient.

E. Statements (1) and (2) TOGETHER are NOT sufficient. Answer:  ----------------------------------------------------------------------------------------------------

3  What is the remainder when the two-digit, positive integer x is divided by 3 ?

(1)     The sum of the digits of x is 5.

(2)     The remainder when x is divided by 9 is 5.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

D. EACH statement ALONE is sufficient.

E. Statements (1) and (2) TOGETHER are NOT sufficient.

Answer:

7 A school administrator will assign each student in a group of n students to one of m classrooms.  If 3 < m < 13 < n, is it possible to assign each of the n students to one of the m classrooms so that each classroom has the same number of students assigned to it?

(1)   It is possible to assign each of 3n students to one of m classrooms so that each classroom has the same number of students assigned to it.

(2)   It is possible to assign each of 13n students to one of m classrooms so that each classroom has the same number of students assigned to it.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

D. EACH statement ALONE is sufficient.

E. Statements (1) and (2) TOGETHER are NOT sufficient.

Answer:

9-19http://forum.chasedream.com/dispbbs.asp?BoardID=22&replyID=516438&id=59242&skin=0

（1）x的可能有14，41，23，32，50；除以3的余数都是2

(2)x=9k+5,x/3=3k+1余2

所以选D

答案：（B）

解释：题目相当于问n能否被m整除。

(1)3n/m为整数，3与m不一定互质，比如3*15/9为整数，15/9却不为整数。

（2）因为3 < m < 13 < n,13是个质数,13与m互质。而13n/m为整数，说明n/m为整数。