我也用了超过五分钟，主要是"要检票前若干分钟就开始排队"...

这句话一开始没理解好。如果理解了开始检票前，旅客的增长率也是常数，那题目就很容易了。

y = rate of passenger growth (person/minute)
x = number of minutes the line had been forming BEFORE admission starts
z = rate of admission
A = how long it takes with 8 windows(answer)

y*(x+40) = 3*40*z ==> x+40 = 120*(z/y)
y*(x+25) = 4*25*z ==> x+25 = 100*(z/y)

(z/y) can be treated as one variable here, so this is easily solved to get the two variables x and (z/y), then just solve:
y(x+A) = 8*A*z ==> x+A = 8*A*(z/y)
to get A

so 处方 was right, 10 minutes

所有跟帖： 

• 啥问题？家有奥数少年的俺，木油看出来。。。 -Yuan- ♀ (0 bytes) () 02/16/2009 postreply 16:13:05

• 排在队里滴=被processed滴，总不总的不知道，是个动态的处理工作 -Yuan- ♀ (0 bytes) () 02/16/2009 postreply 18:01:56