求解一道数学题！谢谢啦

Emilysun · 发表于 2010-12-23 23:21:57

算了半天得不出正确结果，答案选12，求高人指点：
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
A.   4
B.   6
C.   8
D.   10
E.   12

sdcar2010 · 发表于 2010-12-23 23:37:44

category X Y (X + Y)
make w d + 2 d
make 5w/4 ? 3
Rate w/(d +2) w/d 5w/12

Since the combined rate of (X + Y) equals the combination of each individual rate, then [w/(d +2) + w/d] = 5w/12 or [d + (d + 2)]*12 = 5 * d * (d + 2).
So d = 4
To make 5w/4, X alone will take = [5w/4] / [w/6] = 7.5 days

sdcar2010 · 发表于 2010-12-23 23:38:20

LZ, your question should ask for "how many days would it take machine X alone to make 2w widgets!"

Emilysun · 发表于 2010-12-24 17:12:04

Thank you very much for your reply!!
This is a question from the GWD, in the second set. I think you are right. there are also some other questions whose answers are not convinsible...

Emilysun · 发表于 2010-12-24 17:24:02

Sorry, the answer is correct. if d=6, then it will take 6*2=12 days for X alone to finish 2W.

求解一道数学题！谢谢啦

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