嗬嗬

http://news.wenxuecity.com/messages/200902/news-gb2312-794590.html

這裏誰在15分鍾之內沒做出來？ 答案是10分鍾

某車站要檢票前若幹分鍾就開始排隊，每分鍾來的旅客人數一樣多。如果同時開放3個檢票口，那麽40分鍾後檢票口前隊伍恰好消失;如果同時開放4個檢票口，那麽25分鍾後隊伍恰好消失。如果同時開放8個檢票口，那麽隊伍多少分鍾後恰好消失？

set: Passengers started to queue x minutes before check- point opened.
set: y passengers come to check-point per min
set: each checkpoint processes z passengers per min

set: After n minutes, the queue will be cleared if opening 8 counters

3 check-points, 40 min
x*y + 40*y = 3*z*40 ...............(1)

4 check-points, 25 min
x*y + 25*y = 4*z*25 .............(2)

4 check-points, n min
x*y + n*y = 8*z*n ..................(3)

(1)-(2)==> y=4/3*z  ..............(4)
(3)-(2)==> (n-25)*y = 4*z*(2n-25).............. (5)

Combining (4) and (5) ==> (n-25)*4/3z = 4z(2n-25)==>n=10 min




不知道小學算式題怎麽列。