Here’s a long corridor with a moving walkway. Let’s race to the far end and back. We’ll both run at the same speed, but you run on the floor and I’ll run on the walkway, going “downstream” to the far end and “upstream” back to this point. Who will win?
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You’ll win. It’s tempting to predict a tie, as the walkway’s speed will be added to mine on the outbound leg and subtracted on the return … but I’ll spend much more time on the return, so my average speed will suffer.
For the same reason, an airplane can make a round trip more quickly in calm weather than it can when contending with a headwind/tailwind.