Would be simple enough for a rough calibration of the accuracy of the caller’s fuel gauge. Too simple to be a puzzler. At indicated 3/4 tank fill and log the result in miles and gallons. Do it again at 1/2 tank. And again at 1/4 tank. I think one will find the tank is at about 1/3rd full when 1/2 is indicated.
I think MPG will be essentially the same for each fill but there will be more variation for filling at 3/4 because if repeatability in just what is a full tank is ± 1/2 gallon then there will be more error in 4 gallons ± 1/2 than for 12 gallons at 1/4.