Apologies for taking so long to reply!
Here is a 2 page copy from the Jepp JAA book on Powerplants.
Powerplant.pdf - File Shared from Box - Free Online File Storage
It explains how the decrease in temperature (with a constant MAP) and the reduced atmospheric pressure result in more power if a constant MAP and RPM are kept when at higher altitudes.
The constraints don't actually make sense, as 20 GPH at 8000 ft with an equivalent mixture setting will give more power than 20 GPH at sea level. If you allow the mixture setting to be varied, to maintain 20 GPH by enriching the mixture, then there's a continuum of MP vs mixture settings that will meet those constraints.
bookworm... 20 GPH is the mixture setting! Creating a specific power (ie. 45% 65% 75%, etc) requires a specific fuel flow for each. If you create 65% power at 2000', 4000', 12000', etc you will be burning the same fuel at each altitude. You will have a higher TAS at altitude because of less drag due to reduced density. I'm not sure how the constraints on my original post don't make sense.