模考时最后一题，感觉题目有点不明确。题目如下： a hiker walking ar a constant rate of 4 mile per hour is passed by cyclist traveling in the same direction along the same path at a constant rate of 20 miles 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? 答案是20. 有点不理解，那个cyclist在passing那个hiker后就等她，题目的意思貌似再pass后还有一个缓冲，这个缓冲的距离应该就是hiker要赶的距离。计算cyclist离hiker多远也要知道时间。但是pass有蕴含时间(⊙_⊙)？而且那个cyclist还停下来等了5分钟。 没理清思路，希望哪位XDJM帮忙看看给下提醒。 还有一题PREP的，答案我也没弄清楚，感觉应该是100%而不是40%。 题目是：the rate of certain chemical reaction is directly proportional to the spuare of the concentration of chemical A present and inversly proportional to the concentration of chemical B present.If the concentration B is incresed by 100% percent while of the following is closest to keep the reaction rate unchanged? 答案有A increse 100% ,A increse 40% .还有这两个比例decrese的，当然通过题意可以排除decrese如果要保持不变的话。但为什么是40%?0.4^2=0.16啊，100%^2=100%. O(∩_∩)O谢谢各位啦！ -- by 会员 daisy727suming (2010/9/24 19:31:05)
我帮你查了下第一题，你看看一个外国网站的解释~
probably the most efficient way to solve this one is with the concept of relative rates.
start at the time when the cyclist passes the hiker.
since they're both traveling in the same direction, their relative rate - i.e., the rate at which the distance between them is changing - is found by subtraction. so, as long as they're both moving the cyclist is gaining on the hiker at a rate of 20 - 4 = 16 mph.
after the cyclist stops, the hiker is making up that distance at a rate of 4 mph. that's only 1/4 as fast as the relative rate during the first 5 minutes, so it will take the hiker 4 times as long to catch back up. so, 4 x 5 = 20 minutes.
if you wanted to, you could calculate the actual distance between the cyclist and the hiker when the cyclist stops: (16 mi/h)(1/12 h) = d, where "1/12 h" is five minutes. then you could find the time taken by the hiker alone to travel the same distance "d".
you don't have to do this, though, if you realize the 1:4 ratio referenced above
第二题还在看！ | 
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发表于 2010-9-24 19:31:05
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