It's basically the number that along with 43.5 will divide by two to equal approximately 46.5.
Key word there being "approximately". Your solution gives an incorrect result, although it does indeed provide a quick and reasonably accurate (+/- 2%) answer. Nicely done.
However, if the variance in the MPG on the trip out and the overall trip had been greater, your method would yield less accurate results. For instance, if I had gotten 25 MPG on the out leg, and 40 MPG overall, your averaging method would have had considerable error. The correct answer would be
82% higher than the answer your method provides.
tanstaafl.
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