PREP 上有两题不会做，请教！

xiaoniuren

each of the 25 balls in a certain box is either red ,blue,or white and has  a number from 1 to 10 painted on it ,if one ball is to be selected at random from the box , what is the probability that the ball selected will either be white or have an even number painted on it ?

(1) the probability that the ball will both be white and have an even number painted on it is 0

(2) the probability that the ball will be white minus the probability that the ball will have an even number painted on it is 0.2

a certain city with a population of 132,000 is to be divided into 11 voting districts ,an no district is to have a population that is more than 10 perccent greater than the population of any other district,what is the minimum possible population that the least populated district could have

(1) 10,700 (2) 10,800 (3) 10,900 (4) 11,000 (5) 11,100

meowlac

第一个选E么？？？

第二个他说让你求最小的可能值，当其他10个district相等且都是这个最小值的110%时候  符合条件  要想 他最小就要让其他的10个最大~

meowlac

第一题   P（either white or even number)= P(white)+P(even)-P(white and even)       

条件1  告诉了你P(white and even)      

条件2却告诉你P(white)-P(even)

所以求不出来

[此贴子已经被作者于2009-5-6 10:50:27编辑过]

xiaoniuren

第一题答案是E

xiaoniuren

第一题明白了。 第二题还是有点糊涂

看来数学真太差了

还有一题，

at a certain supplier,a machine of type A costs $20,000 and a machine of type B costs $50,000,each machine can be purchased

by making a 20 percent down payment and repaying the remainder of the cost and the finance charges over a period of time ,if

the finance charges are equal to 40 percent of the remainder of the cost,how much less would 2 machines of type A cost than 1

machine of type B under this arrangement?

（1）10，000 （2） 11，200 （3） 12，000 （4） 12，800 （5） 13，200

答案是E

我折腾了半天，还是没有折腾出这个答案。

请神牛再次出现

meowlac

B？

等待nn

[此贴子已经被作者于2009-5-6 12:01:07编辑过]

xiaoniuren

the key is 13,200

xiaoniuren

at a certain supplier,a machine of type A costs $20,000 and a machine of type B costs $50,000,each machine can be purchased

by making a 20 percent down payment and repaying the remainder of the cost and the finance charges over a period of time ,if

the finance charges are equal to 40 percent of the remainder of the cost,how much less would 2 machines of type A cost than 1

machine of type B under this arrangement?

（1）10，000 （2） 11，200 （3） 12，000 （4） 12，800 （5） 13，200

答案是E

这题还是不知道如何得出答案的？