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http://news.wenxuecity.com/messages/200902/news-gb2312-794590.html
这里谁在15分钟之内没做出来? 答案是10分钟
某车站要检票前若干分钟就开始排队,每分钟来的旅客人数一样多。如果同时开放3个检票口,那么40分钟后检票口前队伍恰好消失;如果同时开放4个检票口,那么25分钟后队伍恰好消失。如果同时开放8个检票口,那么队伍多少分钟后恰好消失?
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
不知道小学算式题怎么列。