[原创]几道数学题错了~~搞不明白请高手帮忙

第一题:Bernie has already read 1/5 of his biology homework on Monday night.What fraction of his remaining homework would Bernie have to read on Tuesday night in order to have read 2/3 his original biology assignment?

(A)   7/10   (B)  7/12  (C)  5/12  (D)   2/3 (E) 1/15        我选的是B但是答案是C,但是怎么都不知道为什么

第二题:Two cubes each have 6 sides, which represents 1,2,3,4,5,6, respectively.Put the two cubes on the desk,how many possibilities are there that the sum of the numbers on the two upsides is even?

(A)     9    (B)   6   (C)     18     (D)       24  (E)  12                            我选得是E但答案是C,没搞明白

第三题:A marching band of 240 musicians are to march in a rectangular formation with m rows of exactly n musicians each. There can be no less than 8 musicians per row and no more than 30 musicians per row.How many different rectangular formations are possible?

(A)   5      (B)    4   (C)    6   (D)      7     (E)         8                                我选得是D答案是E,我算出来是8,10,12,16,20,24,30只有7种

请大家帮忙下吧...因为毕竟不直接说,所以希望能说得详细下~~谢谢

  我来说第二题好了：需要两个色子掷出来的数字相加是偶数，则需要两个数字相加是偶数。

  而两个数字相加为偶数，则需要两个奇数相加，或者两个偶数相加。

  在1、2、3、4、5、6中分别有三个奇数和三个偶数，掷出两个都是奇数的情况有3*3=9种；掷出两个都是偶数的同理，9种情况。

  相加，18种情况。

  第一题和第三题，我和你的答案一样，汗，继续等待高人出现~

第三题:A marching band of 240 musicians are to march in a rectangular formation with m rows of exactly n musicians each. There can be no less than 8 musicians per row and no more than 30 musicians per row.How many different rectangular formations are possible?

(A)   5      (B)    4   (C)    6   (D)      7     (E)         8                                我选得是D答案是E,我算出来是8,10,12,16,20,24,30只有7种

还有15

这个可以证明一定是偶数个的，因为允许8和30，而且240不是某整数的平方

第二题:Two cubes each have 6 sides, which represents 1,2,3,4,5,6, respectively.Put the two cubes on the desk,how many possibilities are there that the sum of the numbers on the two upsides is even?

(A)     9    (B)   6   (C)     18     (D)       24  (E)  12                            我选得是E但答案是C,没搞明白

两歌筛子不区分的话，我其实同意你的答案

第一题:Bernie has already read 1/5 of his biology homework on Monday night.What fraction of his remaining homework would Bernie have to read on Tuesday night in order to have read 2/3 his original biology assignment?

(A)   7/10   (B)  7/12  (C)  5/12  (D)   2/3 (E) 1/15        我选的是B但是答案是C,但是怎么都不知道为什么

because bernie have read 1/5, the remaining homework is 4/5. in order to finish 2/3 as assignment, he has to read 2/3 - 1/5 = 7/15, and 7/15 is 7/12 of the remaining homework.

骰子那道题，这样想好了：

2个骰子，都各有1/2 的机会掷到单数/双数。

1）：1单 2单---和为双

2）：1双 2单---和为单

3）：1单 2双---和为单

4）：1双 2双---和为双

因为单双几率一样，所以1+4的几率和2+3的几率是相同的。

因为一共6*6=36种match，所以 双数是18种。

谢谢大家帮忙~~