Once again  do you know the difference between Median and Mean averages? I'm guessing not, at this point. Until you learn the difference, your posts about P&G mean literally nothing.
You might be "reading" the facts, but you're "interpreting" them incorrectly.
Yes, the Median Average of 30 and 20 is 25. The Mean average takes into account the actual acceleration and deceleration rates, and specific times at each speed.
If you were calculating the Mean average, you'd be able to figure out rather quickly that you'll not be averaging 25 MPH if you're using the method correctly, since you're accelerating much faster than you're decelerating. Your graph shows this first hand. Decel obviously takes longer than accel, which skews the average toward the higher side.
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