[求助]2道gmatprep的题

zs8373

is the integer n odd?

1) n is divisible by 3

2) 2n is divisible by twice as many positive integers as n

2的意思不是很明白.答案是b

70, 75,80,85,90,105,105,130,130.130

the list shown consists of the times, in seconds, that i took each of 10 sch

oolchildren to run a distance of 400 on of meters. if the stardard deviatiation of the 10 running times is 22.4 seconds, rouned to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

a) one b) two c) three d) four e )five

答案是b

是不是要算出average(100), 100-22.4=77.6,因为是more than 1 standard deviation below , 77.6-1=76.6 only 70,75<76.6 所以选b

如果是more than 3 standard deviation below ,就应该是77.6-3=74.5就应该选a)

不知道我的理解对不对,请大家帮忙.

julia_ggw

我觉得mm关于1 standard deviation的理解有点问题，more than 1 standard deviation below应该就是指小于100-22.4=77.6的那些数。如果是more than 3 standard deviation below则应该是小于100-22,4*3的那些数

zs8373

多谢,但是第一个题的2n is divisible by twice as many positive integers as n是什么意思?

shaoji

举个例子，如果n＝2是偶数，因数有1，2，两个，但是2n＝4，只有1，2，4三个，

但是如果n＝3是奇数，因数有1，3两个，但是2n＝6，因数就有1，2，3，6四个，所以2n的因数是n的因数2倍时，n一定是奇数。因为偶数的2倍有2这个重合因子。

titatita

以下是引用shaoji在2006-1-9 13:45:00的发言：

举个例子，如果n＝2是偶数，因数有1，2，两个，但是2n＝4，只有1，2，4三个，

但是如果n＝3是奇数，因数有1，3两个，但是2n＝6，因数就有1，2，3，6四个，所以2n的因数是n的因数2倍时，n一定是奇数。因为偶数的2倍有2这个重合因子。

agree,和我得思路一样,还有别得思路吗

seasnow

本质原因是因为2n和偶数的2是重复的。

如果换成3n的话情况就不一样了：就只能判断是否是3的倍数了。

conquergmat

70, 75,80,85,90,105,105,130,130.130

the list shown consists of the times, in seconds, that i took each of 10 sch

oolchildren to run a distance of 400 on of meters. if the stardard deviatiation of the 10 running times is 22.4 seconds, rouned to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

a) one b) two c) three d) four e )five

答案是b

怎么做?

conquergmat

顶

quickgao

算出avg=100

 1 standard deviation below the mean ：即小于100-22.4=77.6的数的个数

只有70，75两个，所以答案是B.

promising

is the integer n odd?

1) n is divisible by 3

2) 2n is divisible by twice as many positive integers as n

2的意思不是很明白.答案是b

2)的意思是2n可以被2n个数字整除,还是说2n的factor的数目是n的factor的数目的2倍啊????