21 people per minute enter a previously empty train station
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20 Jun 2021, 00:35
From 7:00:01 pm to 7:04:01, 84 people enter the first train (21 per
minute). These 84 people will depart at 7:04.
After that, for each 9-minute period, 9(21) = 189 people will enter the next
train. These trains will leave at 7:13, 7:22, 7:31, 7:40, 7:49, 7:58, 8:07, 8:16,
and 8:25.
Since 9 trains each have 189 people and the first train has 84 people, the
average is:
9(189) + 1(84)/10=178.5
Note that the strange time format (minutes and seconds) doesn’t make the
problem any harder—the problem is actually clearer because people board the
train at 1 second after each minute, whereas the train leaves at the beginning
of each minute.