[求助]这道数学题不会

不好意思，，，偶的数学比较烂

这道题不会啊，，，

if car X followed car Y across a certain bridge that is 1/2 mile long,how many seconds did it take car X to travel across the bridge?

(1)car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge

(2)car Y traveled across the bridge at a constant speed of 30 miles per hour

这道题不会啊，，，看了OG的解释也没明白

OG对（1）的解释是：if car X drove onto the bridge 3 seconds after car Y drove onto the bridge and drove off the bridge 2 seconds after car Y ,then car X took 1 second less to across the bridge than car Y

我就是这里不明白啊 为什么开上去3秒开下来2秒 就怎么会比Y少一秒呢。。。。

这道题也不会。。。

what is the maximum number of rectangular blocks,each with dimensions 12 centimeters by 6 centimeters by 4 centimeters,that will fit inside rectangular BOX X?

1)when box x is filled with the blocks and rests on a certain side,there are 25 blocks in the bottom layer.

2)the inside dimensions of BOX X are 60 centimeters by 30centimeters by 20 centimeters.

看了OG的解释，，没怎么看懂，，，

就是X跟在Y后边上桥，X上到桥上时Y已经上桥3秒钟了，X下桥时Y刚下桥2秒。就说明X在桥上车速比Y快，所以过桥时间节省了1秒。

比如说：Y在10：01：01秒上桥，用了10秒过桥于10：01：11秒下桥。X跟在后边慢3秒上桥即10：01：04秒上桥，下桥比Y省了1秒也就是用9秒过桥，即10：01：13秒下桥。

谢谢LS

二楼的题目有人会吗。。。

应该选B吧? 2)alone就可以了,

盒子尺寸   60(5*12) X 30(5*6) X 20(5*4) cm

砖块尺寸           12   X         6   X         4

数量应该         5       X    5        X      5     =125块

关键, 其他塞法,塞不满箱子.

这样, 1)就充分不必要了.

以下是引用小肚兜佛祖在2009-7-6 15:00:00的发言：

不好意思，，，偶的数学比较烂

这道题不会啊，，，

if car X followed car Y across a certain bridge that is 1/2 mile long,how many seconds did it take car X to travel across the bridge?

(1)car X drove onto the bridge exactly 3 seconds after car Y drove onto the bridge and drove off the bridge exactly 2 seconds after car Y drove off the bridge

(2)car Y traveled across the bridge at a constant speed of 30 miles per hour

这道题不会啊，，，看了OG的解释也没明白

OG对（1）的解释是：if car X drove onto the bridge 3 seconds after car Y drove onto the bridge and drove off the bridge 2 seconds after car Y ,then car X took 1 second less to across the bridge than car Y

我就是这里不明白啊 为什么开上去3秒开下来2秒 就怎么会比Y少一秒呢。。。。

这道题的意思是x和Y都通过一座1/2mile的桥，问下面两个条件能否得知X过桥用了多少时间

1. X在Y上桥后3秒上桥，下桥的时候比Y慢两秒。言下之意就是X在桥上的时间比Y短一秒。 1单独显然不充分

2.告诉了Y的速度，已知路程，可得Y通过的时间。俺懒，就不单位换算了。2显然也不充分

联合1，2，知道Y过桥时间，而X用时比Y少1秒，则X可以确定。选 C

1.只知道最下面一层是25个，而不知道小的长方体是顺着哪条边放的，故答案不确定

2.知道大长方体的长宽高和小长方体的长宽高，就一定有一种最经济的方法使小的个数最大化。确定