来一道GMAT_PREP难题

bosfashion

A certain jar contains only b black marbles, w white marbles, and r red marbles, if one marble is to be chosen at random from the jar, is the probability that the marble chosen will be red greater than the probability that the marble chosen will be white.

(1) r / (b+w) > w / (b+r)

(2) b-w > r

答案选A, 我不明白怎么算的

请高人指点我一下，请各位讲的详细一些，谢谢了

bth

We need to state that R>W
1) r/(b+w) > w/(b+r), add 1 on each side and solve the fraction
it will be (r+b+w)/b+w > (r+b+w)/b+r => from this we can say r > w

2) can't say anythinf about relation between r&B...insuff

Hence, A is the answer.

Am I doing any wrong??

mymengming

请标明题号，方便以后查找
还有，发贴前请先查询，查询方法：

http://forum.chasedream.com/dispbbs.asp?boardID=22&ID=329391&page=1
这里有大部分讨论汇总链接
找到prep
然后
http://forum.chasedream.com/dispbbs.asp?BoardID=22&replyID=1404763&id=150752&skin=0
用crtl + f找到你的题目
然后去相应楼层找到解答，谢谢

bth

I think some elaboration is required:

r/(b+w) > w/(b+r)

r/(b+w) + 1 > w/(b+r) + 1

r/(b+w) + (b+w)/(b+w) > w/(b+r) + (b+r)/(b+r)

r/(b+w) + (b+w)/(b+w) > w/(b+r) + (b+r)/(b+r)

(r+b+w)/(b+w) > (r+b+w)/(b+r)

1/(b+w) > 1/(b+r) [Divided both sides by (r + b + w)]

b+r > b+w

r > w

Note: Don't worry about the inequality being switched because in this probelm you know that you'll always be working with positive integers. [You can't have negative number of marbles]

ernestnie

bth is good! Li Hai.