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A plane starts the day at the airport, does an even number of flights back and forth, and then one last flight and ends it at another airport. Repeat the next day but in reverse.
But it is the average number, not the number at exactly that one day.
The way I described it, there would be an odd number of flights every day, so the average will also be odd.
Imagine there was only one flight. Day 1 it leaves Edinburgh and lands at Heathrow. Day 2 it leaves Heathrow and lands back in Edinburgh. Then repeat again. There is exactly one flight every day, so the average is odd.
Every plane lands and starts, that are 2 fights. Any daily etc. irregularities cancel out over a year. What could still happen is something like a plane landing on 31.12. but not taking off before it is 01.01. Considering that… alright, it does not need to be an even number. And those are exact numbers, not averages.