Prep07数学错题疑问集ps（2）【至5.19未解答状态0/4】

灵狩Icy

1. Working alone at its own constant rate, a machine seals k cartons in 8 hours, and working alone at its own constant rate, a second machine seals k cartons in 4 hours.  If the two machines, each working at its own constant rate and for the same period of time, together sealed a certain number of cartons, what percent of the cartons were sealed by the machine working at the faster rate?【已解答D 谢谢 不如前】
(A) 25%
(B) (33+1/3)%
(C) 50%
(D) (66+2/3)%
(E) 75%
算出答案为2/3, 单位是%，因此分子要*100。

2. a hiker walking at a constant rate of 4 miles per hour is passed by a cyclist traveling in the same direction along the same path at a constant rate of 20 mile per hour.the cyclist stops to wait for the hiker 5 minutes after passing her,while the hiker continues to walk at her constant rate.how many minutes must the cyclist wait until the hiker catches up? 【已解答20min 谢谢 不如前】
题目说的是自行车超过步行者5分钟后，停下来要等多久。步行速度：4/60  自行车速度：20/60
超过5分钟后，自行车走，行人同时也走了。因此两人相差(20-4)/60*5 mile。再除以步行的速度。

3. A contest will consist of n questions, each of which is to be answered either "True" or"False." Anyone who answers all n questions correctly will be a winner. What is the leastvalue of n for which the probability is less than 1/1000 that a person who randomly guessesthe answer to each question will be a winner?
【已解答10 谢谢 不如前】
每次蒙对的概率为1/2，十次都蒙对的概率为 (1/2)^n<1/1000

4. During a trip on an expressway, Don drove a total of x miles.  His average speed on a certain 5-mile section of the expressway was 30 miles per hour, and his average speed for the remainder of the trip was 60 miles per hour.  His travel time for the x-mile trip was what percent greater than it would have been if he had traveled at a constant rate of 60 miles per hour for the entire trip?【已解答E 谢谢 不如前】
(A)  8.5%
(B)  50%
(C)  x/12%
(D) 60/x%
(E) 500/x%
（原本的时间-匀速时间）/匀速时间*100

不如前

灵狩Icy 发表于 2015-5-19 08:43
咦，我还是晕，为啥不是2/3 *%，而是*100 呢？？

比如你算出来等于0.2，答案选的是20%，也就是0.2*100

不如前

因为那五分钟人也在走- -   所以差距不是20/60*5，而是（20-4）/60*5

不如前

3.（1/2)^2<1/1000
4.原来用的时间t1=5/30+(x-5)/60
后来用的时间t2=x/60
100*(t1-t2)/t2

不如前

灵狩Icy 发表于 2015-5-19 22:07
第四题又是用到了*100的情况，刚刚你给我讲过~~明白啦

啊我打错了   是（1/2)^n     乱蒙对的概率是二分之一，n次都对的概率就是n次方，然后要小于1/1000，至少有10道题概率是1/1024

不如前

注意单位，是%
所以是2/3*100=66+2/3

灵狩Icy

不如前 发表于 2015-5-19 08:19
注意单位，是%
所以是2/3*100=66+2/3

咦，我还是晕，为啥不是2/3 *%，而是*100 呢？？

灵狩Icy

不如前 发表于 2015-5-19 09:20
比如你算出来等于0.2，答案选的是20%，也就是0.2*100

原来是这样！！谢谢宝贝儿！！！！

不如前

灵狩Icy 发表于 2015-5-19 10:04
原来是这样！！谢谢宝贝儿！！！！

不谢不谢~     看到你发了好多贴好认真!!         感觉零食已经在向我招手了哈哈哈哈  
加油！！

灵狩Icy

不如前 发表于 2015-5-19 10:11
不谢不谢~     看到你发了好多贴好认真!!         感觉零食已经在向我招手了哈哈哈哈  
加油！！ ...

26号考完告诉你！！！只是我的数学还是每次都会错10道左右。。OTZ我个渣渣。。

灵狩Icy

不如前 发表于 2015-5-19 21:06
因为那五分钟人也在走- -   所以差距不是20/60*5，而是（20-4）/60*5

咦，原来是这样！！