大大 解救math题

finger1234

1.If a committee of 3 people is to be selected from among 5 married couples so that the committee does not include two people who are married to each other, how many such committees are possible?

Key:80 为什么？ 不是C35×2＝20？

2.Last Sunday a certain store sold copies of Newspaper A for $1.00 each and copies of Newspaper B for $1.25 each, and the store sold no other newspapers that day. If r percent of the store's revenues from newspaper sales was from Newspaper A and if p percent of the newspapers that the store sold were copies of newspaper B, which of the following expresses r in terms of p?

Key:400p/(500-p)  我怎么也算不到这个答案

3.Last year the price share of Stock X increased by k percent and the earnings per share of  Stock X increased by m percent, where k is greater than m. By what percent did the ratio of  price per share to earnings per share increase, in terms of k and m?

Key:[100(k-m)]/(100+m)%  困境同上

4. A school administrator will assign each student in a group of n students to on e 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 the smae number of students assinged to it?

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

Key:B why? What is that mean?

5. If the probability is 0.54 that Stock A will increase in value during the next month and the probability is 0.68 that Stock B will increase in value during the next month, what is the greatest possible value for the probability that neither of these two events will occur?

Key:0.32     为什么不是1-0.54*0.68  neither的意思不是两个都不会发生吗?

6. Six card numbered from 1 to 6 are placed in an empty bowl. First one card is drawn and then put back into the bowl; then a second card is drawn. If the cards are drawn at random and if the sum of the numbers on the cards is 8, what is the probability that one of the two cards drawn is number 5?

Key: 2/5 why?

ada3063

第一题; c10,3-c5,1*c8,1

KennyChenzju

Answer NO.3: Set P as share price, E as earning per share, x as percent of increase of "price per share to earnings per share", which can also be recognized as shares. Therefore,

P*(1+k%)/S(1+m%) = P*(1+x%)/S   =>  x= 100(k-m)/(100+m)

Here, x represents the percent, not including "%". If it asks for the "percentage", you should add "%" after the formula

For NO.2, I didn't get that answer either... Anyone with any ideas?

KennyChenzju

Answer to NO.6: the possible solutions of taking two sequential cards that sumed up at 8 are:

(2,6), (6,2), (3,5), (5,3) and (4,4)

Therefore, the chance taking one of them numbered at 5 is 2/5.

finger1234

thank you so much, But any one can answer the rest?

xinxian

1. Total ways to select 3 people from 5 couples (10 people) = 10C3 = 120

    Ways to ensure that a couple is selected to the committee = 5C1*8C1 = 40

    

    Ways to ensure that no couple is selected to the committee = 120-40=80

xinxian

Let the total newspaper sold be 100.

    

    Newspaper A sold = p

    Newspaper B sold = 100-p

Total revenue = p*1+(100-p)*1.25

So revenue from paper A = p*1 = p

Revenue percentage from paper A = (p/(p+(100-p)*1.25))*100

This is equal to r

thus

(p/(p+(100-p)*1.25))*100 = r

    

=> r = 100p/(125-0.25p)

Multiplying by 4 in numerator and denominator on RHS, we get

r = 400p/(500-p)

xxy870128

who got NO5? i can not find it

finger1234

but p is the percent of the newspapers that the store sold were copies of newspaper B,so Newpaper A sold should be 100-p not p, that is why I can not get to the answer.