求助:GMATprep几道数学，谢谢！！

请大家帮帮忙，有几道GMATprep的数学实在想不明白

1）In a certain senior class, 72%of the male students and 80% of the female students have applied to college. What fraction of the students in the senior class are male?

   (I) There are 840 students in the senior class

   (II) 75% of the students in the senior class have applied to college

答案Ｂ（想不通，为什么不选Ｃ，请指教）

２）Store S sold a total of 90 copies of a certain book during the seven days of last week, and it sold different numbers of copies on any two of the days. If for the seven days, store S sold the greatest number of copies on Saturday and second greatest number of copies on Friday, did store S sell more than 11 copies on Friday?

   (I) Store S sold 8 copies on Thurday

   (II) Store S sold 38 copies on saturday

答案Ｂ　（想不通，只有Ｂ怎么能确定，为什么不是Ｃ？）

３）On a number line,the segment from 0 to 1 has been divided into fifths(as indicated by large tick marks), and also into sevenths (as indicated by the small tick marks). What is the least possible distance between any two of the tick marks?

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答案：1/35  (实在算不出来，怎么算都是2/35，请帮帮我吧~）

4）If (&) denotes one of two arithmetic operations, addition or multiplication and if K is an integer, what is the value of ３　（＆）Ｋ？

     (I) 2 (&) k=3

     (II) 1 (&) 0 = k

答案　Ａ　（请指教 I 既然对了，II为什么不对呢？）

5）A store purchased 20 coats that each cost an equal amount and then sold each of the 20 coats at an equal price. What was the store's gross profit on the 20 coats?

    (I) If the selling price per coat had been twice as much, the store's gross profit on the 20 coats would have been $2400

     (II) If the selling price per coat had been $2 more, the store's gross profit on the 20 coats would have been $440.

答案Ｂ　（　为什么不选Ｄ呢？（Ｉ）哪里有问题呢？）

非常非常感谢～～～～

1）In a certain senior class, 72%of the male students and 80% of the female students have applied to college. What fraction of the students in the senior class are male?

   (I) There are 840 students in the senior class

   (II) 75% of the students in the senior class have applied to college

答案Ｂ（想不通，为什么不选Ｃ，请指教）

female x人,male y人.

72%*x+80%*y=75%*(x+y)  ==>  可以得出x和y的关系,求的是y/(x+y) 可以得出答案

２）Store S sold a total of 90 copies of a certain book during the seven days of last week, and it sold different numbers of copies on any two of the days. If for the seven days, store S sold the greatest number of copies on Saturday and second greatest number of copies on Friday, did store S sell more than 11 copies on Friday?

   (I) Store S sold 8 copies on Thurday

   (II) Store S sold 38 copies on saturday

答案Ｂ　（想不通，只有Ｂ怎么能确定，为什么不是Ｃ？）

周六卖了38,剩下六天总共52,各天分别为6,7,8,9,10,11也才51,所以肯定有大于11的

4）If (&) denotes one of two arithmetic operations, addition or multiplication and if K is an integer, what is the value of ３　（＆）Ｋ？

     (I) 2 (&) k=3

     (II) 1 (&) 0 = k

答案　Ａ　（请指教 I 既然对了，II为什么不对呢？）

(I) 如果& 表示加,则K=1,得出答案;如果&表示乘,则k=1.5 不是整数.所以(I)对的

(II)如果&表示加,则K=1,得出一个答案;如果&表示乘,则K=0,得出另一个答案,所以不确定

5）A store purchased 20 coats that each cost an equal amount and then sold each of the 20 coats at an equal price. What was the store's gross profit on the 20 coats?

    (I) If the selling price per coat had been twice as much, the store's gross profit on the 20 coats would have been $2400

     (II) If the selling price per coat had been $2 more, the store's gross profit on the 20 coats would have been $440.

答案Ｂ　（　为什么不选Ｄ呢？（Ｉ）哪里有问题呢？）

(I)没给出具体金额吧

(II)数字对嘛?没看懂

答案：1/35  (实在算不出来，怎么算都是2/35，请帮帮我吧~）

最小距离： 3/7-2/5=1/35

5）A store purchased 20 coats that each cost an equal amount and then sold each of the 20 coats at an equal price. What was the store's gross profit on the 20 coats?

    (I) If the selling price per coat had been twice as much, the store's gross profit on the 20 coats would have been $2400

     (II) If the selling price per coat had been $2 more, the store's gross profit on the 20 coats would have been $440.

答案Ｂ　（　为什么不选Ｄ呢？（Ｉ）哪里有问题呢？）

Gross Profit=sales-cost

要求 GP=20*(p-c)=20p-20c

(I)P'=20(2p-c)=40p-20c=2400=>算不出20p-20c

(II)p''=20(p+2-c)=20p-20c+40=440=>20p-20c=200  可以算出~ 所以B