GWD数学请教，数学好的朋友们请指点一下，万分感谢

Q30.  C ??      C是如何得出的呢？

Yesterday it took a certain plane 3 hours to fly from City A to

City B at an average speed of 400 miles per hour. Today the

same plane flew from City A to City B along the same route at

an average speed of 450 miles per hour. How many more

yesterday than it took today?

A.      10

B.      15

C.      20

D.     25

E.      30

2.  Of the three-digit positive integers that have no digits equal

to zero, how many have two digits that are equal to each

other and the remaining digit different from the other two?

A.      24

B.      36

C.      72

D.     144

E.      216

answer    E项是如何算出的呢

to zero, how many have two digits that are equal to each

other and the remaining digit different from the other two?

A.      24

B.      36

C.      72

D.     144

E.      216

answer    E项是如何算出的呢

A.     0.40x

B.     1.20x

C.     1.40x

D.     1.60x

E.      3.20x

A certain club has 20 members.  What is the ratio of the member of 5-member committees that can be formed from the members of the club to the number of 4-member committees that can be formed from the members of the club?

A.     16 to 1                      

B.     15 to 1

C.     16 to 5

D.     15 to 6

E.      5 to 4

Q30.  C ??      C是如何得出的呢？

Yesterday it took a certain plane 3 hours to fly from City A to city B at an average speed of 400 miles per hour. Today the same plane flew from City A to City B along the same route at an average speed of 450 miles per hour. How many more yesterday than it took today?

-我猜最后的问题是How many more 分钟 yesterday吧
昨天用的时间-今天用的时间就可以了
 

2.  Of the three-digit positive integers that have no digits equalto zero, how many have two digits that are equal to each other and the remaining digit different from the other two?

-排列组合，_ _ _，先算可能出现的组合9*8=72，然后有AAB,ABA,BAA三种排列，72*3

Q11:    C  ?? 请教过程A certain business produced x rakes each month form November through February and shipped x/2 rakes at the beginning of each month from March through October.  The business paid no storage costs for the rakes from November through February, but it paid storage costs of $0.10 per rake each month from March through October for the rakes that had not been shipped.  In terms of x, what was the total storage cost, in dollars, that the business paid for the rakes for the 12 months form November through October?
-从11月-2月，生产了4X一共，不付costs
从3月-10月，运输X/2每月，所以每月的cost依次是3.5X,3X,2.5X,2X,1.5X,1X,0.5X,相加等于14X,所以付费1.4X

 
D ????  请教解题过程
A certain club has 20 members.  What is the ratio of the member of 5-member committees that can be formed from the members of the club to the number of 4-member committees that can be formed from the members of the club?
-排列组合啊，MM你是不是阅读有困难？
C（上5下20）/C（上4下20）=(20*19*18*17*16/5*4*3*2*1)/(20*19*18*17/4*3*2*1)

如果看不懂，就搜索一下以前的帖子哈，我只能给你说下解法，我感觉这些题不难，可能都是因为你没有读懂

3.5X,3X,2.5X,2X,1.5X,1X,0.5X,

LS这个是怎么算的呢？

这个题说的是存放东西要给的钱，存了多少就给多少

到2月底的时候，存货是4X,

3月要ship出去0.5X，所以要交3.5X

4月要ship出去0.5X，所以要交3X

…

就这个意思