Open a ticket gate and pass through 20 units in 20 minutes.
Open two ticket gates and pass 2×8= 16 in 8 minutes.
The number of people who are four units short is the new queue number when the difference between 8 minutes and 20 minutes is 12 minutes.
So there are 4÷ 12= 1/3 units in line every minute.
Therefore, 1/3×20=20/3 of the 20 units passing through a ticket gate is the new arrivals in the queue.
So there were 20-20/3=40/3 stations waiting in line before the check-in began.
There are now three ticket gates, of which 1/3 is used to increase the number of people waiting in line, just to ensure that newcomers can pass in time; The remaining 8/3 units are used to transfer the original 40/3 units, so it takes 40/3÷8/3=5 (minutes).