请教3道数学题

mijianture

Q30:
Joanna bought only $0.15 stamps and $0.29 stamps.How many $0.15 stamps did she buy?
(1)She bought $4.40 worth of stamps.
(2)She bought an equal number of $0.15 stamps and $0.29 stamps.  （why it is suffient ? ）



Q25:
A photographerwill arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in front of someone in the second row.The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her.How many such arrangements of the 6 people are possible?


A.5
B.6
C.9
D.24
E.36

Q20:
If x > 0, then 1/[√(2x)+√x] =

A.1/√(3x)
B.1/[2√(2x)]
C.1/(x√2)
D.(√2-1)/√x
                            E.   (1+√2)/√x

题目不难，大家莫见笑，望能者指点，多谢

sdcar2010

Q30:
Joanna bought only $0.15 stamps and $0.29 stamps.How many $0.15 stamps did she buy?
(1)She bought $4.40 worth of stamps.
(2)She bought an equal number of $0.15 stamps and $0.29 stamps.

(1) 440 = 15*x + 29*y   Answers are: x =10, y = 10; sufficient.

AAAAAAAaa

kikisunflower

Q30中（1）（2）都分别sufficient，是不是该选D哦？

sdcar2010

A photographerwill arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in front of someone in the second row.The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her.How many such arrangements of the 6 people are possible?

Let's assume the 6 people are ABCDEF in ascending height, A is the shortest;  F is the tallest. And the 6 positions are:
1 2 3  (2nd row)
4 5 6  (1st row)

Apparently, F has to be in position 3 and A has to be in postion 4:
1 3 F
A 5 6

Then B has to be in position 5 or position 1. Let's do position 1 first.
B 3 F
A 5 6
Where could C go? Positions 3 or 5:
B C F
A D E  Wrong because D is in front of C.

B D F
A C E

B E F
A C D

These are 3 2 possibilies when B is in position 1.

There are 3 more possibilities when B is in position 5.  Do you know what are they?

sdcar2010

Q30中（1）（2）都分别sufficient，是不是该选D哦？

-- by 会员 kikisunflower (2011/3/26 11:31:17)

Why is (2)  sufficient? How many $0.15 stamps?

sdcar2010

If x > 0, then 1/[√(2x)+√x] = [[√(2x)-√x]/x = DDDDDDDDDDDDDDDDDd

kikisunflower

恩，昏了，选A

mijianture

Q30中（1）（2）都分别sufficient，是不是该选D哦？

-- by 会员 kikisunflower (2011/3/26 11:31:17)

Why is (2)  sufficient? How many $0.15 stamps?

-- by 会员 sdcar2010 (2011/3/26 11:35:57)

谢谢大家帮忙，这道题我也搞不懂，GWD答案是D

mijianture

A photographerwill arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in front of someone in the second row.The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her.How many such arrangements of the 6 people are possible?

Let's assume the 6 people are ABCDEF in ascending height, A is the shortest;  F is the tallest. And the 6 positions are:
1 2 3  (2nd row)
4 5 6  (1st row)

Apparently, F has to be in position 3 and A has to be in postion 4:
1 3 F
A 5 6

Then B has to be in position 5 or position 1. Let's do position 1 first.
B 3 F
A 5 6
Where could C go? Positions 3 or 5:
B C F
A D E

B D F
A C E

B E F
A C D

These are 3 possibilies when B is in position 1.

There are 3 more possibilities when B is in position 5.  Do you know what are they?

-- by 会员 sdcar2010 (2011/3/26 11:34:32)

不知道，GWD的答案是A（5）

kikisunflower

（2）肯定不sufficient，在单独看（2）的时候千万不要受（1）的影响……就当（1）不存在