13/3 i belive i just subtracted it not sure if its right hoped i helped
Initial amount of fuel in the tank = [tex]\frac{18}{3}[/tex] gallons
Amount of fuel used = [tex]\frac{5}{3}[/tex]
Amount of fuel left = [tex]\frac{18}{3}-\frac{5}{3}[/tex]
= [tex]\frac{13}{3}[/tex] gallons are left
