两道prep maths向各位请教！ 急！

acc

    

90.   DS

    

On Jane's credit
card account, the average daily balance for a 30-day billing cycle is the
average (arithmetic mean) of the daily balances at the end of each of the 30
days.  At the beginning of a certain
30-day billing cycle, Jane's credit card account had a balance of $600.  Jane made a payment of $300 on the account
during the billing cycle.  If no other
amounts were added to or subtracted from the account during the billing cycle,
what was the average daily balance on Jane's account for the billing cycle?

    

 

    

(1)  Jane's payment was credited on the 21st day
of the billing cycle.

    

 

    

(2)  The average daily balance through the 25th
day of the billing cycle was $540.

    

 

    

D

这道题怎么就看不明白呢 ？请热心朋友解释下，还有第二个是怎么算出来的呢？

    

42.   ＤＳ

    

John and Mary own
shares of stock in a certain company. 
Does John own more shares of the company's stock than Mary?

    

 

    

(1) Mary owns more
than 500 shares of the company's stock.

    

 

    

(2) The number of
shares of the company's stock that John owns is 400 less than twice the number
of shares of the company's stock that Mary owns.

    

 

    

C

这题我是这样想的：（1）M>500;(2)J=2M-400  (1)+(2)=〉J>2*500-400=600,即J>600
但这并不能推出M与J的关系啊？M可以为700，J也可以为650阿？那么Ｍ＞Ｊ；若Ｍ＝６００，Ｊ＝８００，那么Ｊ＞Ｍ

    

117.  PS

    

A certain office
supply store stocks 2 sizes of self-stick notepads, each in 4 colors:  blue, green, yellow, or pink.  The store packs the notepads in packages that
contain either 3 notepads of the same size and the same color or 3 notepads of
the same size and of 3 different colors. 
If the order in which the colors are packed is not considered, how many
different packages of the types described above are possible?

    

 

    

(A)  6

    

(B)  8

    

(C)  16

    

(D)  24

    

(E)  32

    

C

这一题怎么也算不出这个答案，我算出的是24

请各位热心G友指教，多谢各位！

        

    

还有两星期就考了，Maths怎么这么差，555555555

stella_qj

3.

 3 notepads of the same size and the same color : 2*4=8 ;

3 notepads of the same size and of 3 different colors: 2*C(3,4)=8

so 8+8=16 

chenleo

42题的解法如下：

M>500, J=2M-400

J=M+(M-400)

因为M>500,所以M-400>0

故 J>M

很简单吧

acc

谢谢两位G友相助，看得很明白
可还有一题呢？
等待中。。。

jasoncz1

On Jane's credit card account, the average daily balance for a 30-day billing cycle is the average (arithmetic mean) of the daily balances at the end of each of the 30 days.  At the beginning of a certain 30-day billing cycle, Jane's credit card account had a balance of $600.  Jane made a payment of $300 on the account during the billing cycle.  If no other amounts were added to or subtracted from the account during the billing cycle, what was the average daily balance on Jane's account for the billing cycle?
(1)  Jane's payment was credited on the 21st day of the billing cycle.
(2)  The average daily balance through the 25th day of the billing cycle was $540.

她帐号里就动了一次.
1).   从21-30号她里面每天balance300, 前20天是600. 很清楚, (600x20+300x10)/30=average daily balance.
2).   一看知道25天里肯定有一天是付了那credit300, 不然的话average daily balance是600.  设x为credit前的天数, 就是每天balance600的天数.  600x+300(25-x)=540*25. x=20.  就知道300块是21日付的.然后
可以算average daily balance, 和option 1一样.   

[此贴子已经被作者于2007-7-20 23:56:39编辑过]

acc

明白了，谢谢谢谢！