Question 1101350
check 13 gallons
13*12.50+25*31.50=(13+25)*25
162.5+787.5=38*25
950=950
ok 13 gallons

check 17 gallons
17*12.50+25*31.50=(17+25)*25
212.5+787.5=42*25
1000=1050
not ok 17 gallons

Stan's error was 315-250=65 not 85
125x = 85*25 should have been
125x = 65*25
5x=65
x=13