Increasing power required with altitude.. what's a good plain english explanation?

Denny writes:

Uhhhhh, I hate to bust anyones balloon, BUT, more throttle at higher
altitude does not mean you are getting more power than you were
getting with less throttle at lower altitude...

Indeed, if you work out the numbers, you'll find that you can fly with
less power at higher altitudes, even though the throttle may be set
higher.

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