请教PREP数学题

woodlittle

 

152.

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

答案是A

140.

A manufacturer produced x percent more video cameras in 1994 than in 1993 and y percent more video cameras in 1995 than in 1994.  If the manufacturer produced 1,000 video cameras in 1993, how many video cameras did the manufacturer produce in 1995 ?

(1) xy = 20

(2) x + y + xy/100 = 9.2

答案是B

麻烦各位告诉我该怎么做？不甚感激！！

BruceNornia

152 题目是这样的：

 

在jar里面拿一块marble，那么根据题目 拿到red的概率为r/(r+w+b); 拿到blue的概率为b/(r+w+b); 拿到white的概率为w/(r+w+b);

 

如果说要拿到红色的概率大于拿到白色的 需要 r/(r+w+b)> w/(r+w+b) 也就是说r > w 即题目根本上是要判断w和r的大小

 

对于条件1  r/b+w >w/b+r => r/b+w - w/b+r>0 => r(b+r) - w(b+w) > 0 => rr + rb - wb - ww >0 =>  b(r-w) + (r-w) (r+w) >0 => (r-w)(b+r+w) >0

可以看出b+r+w一定是大于0的 所以 r-w>0 r>w 条件1满足

 

对于条件2 b - w>r只能判断出b > r b > w 对于r和w的大小关系不能判断，所以条件2不满足

 

所以选择A

BruceNornia

140题目是这样的...

94年的产量比94年多x% 95年的产量比94年多y%

93年产量为1000

所以94年的产量为1000(1+x%) 而95年的产量为[1000(1+x%)](1+y%) = 1000 (1+x%)(1+y%)=1000 (1+x%+y% + x%y%)

上面的式子中只要能算出(1+x%+y% + x%y%)就能判断题目问题

 

条件1 xy = 20 不够，因为xy = 20 只能判断出X + Y的下限 不能得到具体数字 所以1不行

条件2 x + y + xy/100 = 9.2 =〉x%+y% + x%y% = 9.2% =>  1+x%+y% + x%y% = 1.92 所以2成立

 

选择B

BruceNornia

btw 顺便问问... 大家都用的什么数学复习材料阿

我只有GWD ...别的都没有...谁愿意share下么？

OceanBleu

152. The probability of red: P(r)=r/(b+w+r), and the white P(w)=w/(b+w+r), so if P(r)>(w), then r>w.

(1).r/b+w>w/b+r,-->(r-w)(1-1/b)<0 , because b is positive integer, so (r-w)<0.

(2) b-w>r  --->b>w+r, could not get the result r>w.

so the answer should be A

140.From the information, we could get V94=V93(1+x%), and V95=V94(1+y%), so V95=V93(1+x%)(1+y%)=V93(1+x+y+xy%)%, so (2) i enough. 

woodlittle

 thank!