prep2里的一题 +2

crackingkiko

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

The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 meters.  If the standard deviation of the 10 running times is 22.4 seconds, rounded 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

2.c and d are interger,is c even?

(1) c(d+1)=even

(2) (c+2)(d+4)=even

3. 4^17-2^28的质因子是什么?

答案是7 怎么求的?

谢谢

[此贴子已经被作者于2008-8-11 16:44:13编辑过]

laipe0704

題目說到這十個學生跑出的標準差值是22.4(取到十分位)，mean(算術平均數)可算出為100
求1 standard deviation below the mean of the 10 running times，100-22.4=77.6，十個數中below77.6有兩個
ANS：Two，70&75。

crackingkiko

以下是引用laipe0704在2008-8-11 16:20:00的发言：
題目說到這十個學生跑出的標準差值是22.4(取到十分位)，mean(算術平均數)可算出為100
求1 standard deviation below the mean of the 10 running times，100-22.4=77.6，十個數中below77.6有兩個
ANS：Two，70&75。

谢谢明白了.....原来我没读懂题...