问两类最不会的数学题：概率GWD-1-Q2,工作问题GWD-1-Q26

beijin111 · 发表于 2010-4-18 22:21:47

Q2:
A certain roller coaster has 3 cars, and a passenger is equally likely to ride in any 1 of the 3 cars each time that passenger rides the roller coaster. If a certain passenger is to ride the roller coaster 3 times, what is the probability that the passenger will ride in each of the 3 cars?

A.   0
B.   1/9
C.   2/9
D.   1/3
E.   1

Q26:
Running at their respective constant rates, machine X takes 2 days longer to produce w widgets than machine Y. At these rates, if the two machines together produce 5/4 w widgets in 3 days, how many days would it take machine X alone to produce 2w widgets?

A.   4
B.   6
C.   8
D.   10
E.   12

谢谢~~

beijin111 · 发表于 2010-4-18 22:23:02

以上一个选C，一个选E

jtrchina · 发表于 2010-4-18 23:08:16

Here are the processes through which you can reach the correct answers:
Q2:C(3)(1)*(1/3)*C(2)(1)*(1/3)*C(1)(1)*(1/3)=2/9

Q26:Assign the rate of work,ie. efficiency of X to x,that of Y to y,here is the group of equation:
(1)w/x-w/y=2
(2)5w/(4x+4y)=3
put them together and get 3x=2y
finally put such equation as mentioned above to (1) and get w/3x=2 =>2w/x=12,consistant with E
Hope it helps

lindaaniu · 发表于 2010-5-23 17:34:23

解不出2x=3y啊。。。

小货舱 · 发表于 2010-5-23 19:24:58

上面那题我也不明白, 顶上去给牛牛看看

序列 · 发表于 2010-5-23 20:01:00

leonfine · 发表于 2010-5-23 23:39:09

会得到一个5W^2-34WX+24X^2=0 的方程
即(5W-4X)(W-6X)=0
可证X=5/4W 速度太快不合要求，所以只有X=1/6W

leonfine · 发表于 2010-5-23 23:41:39

前一道题这么想：
第一次上车随便上任何一个车都可以
第二次上车一定要上到非第一次的车（即另外两辆车，三中选二）：可能性2/3
第三次一定要上到非前两次上过的车（三中只能选一）：可能性：1/3
于是1*(2/3)*(1/3) = 2/9

序列 · 发表于 2010-5-26 02:00:19

第2题
x=y+2
(3w/x)+(3w/y)=5w/4 -> 3/x +3/y = 5/4

然后 y=6, x = 4, 2y=12

问两类最不会的数学题：概率GWD-1-Q2,工作问题GWD-1-Q26

所属分类: GMAT考试

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