Question 815279
Let T = tank capacity.


{{{T/2-T/3=3}}}
The difference between half a tank and one-third of tank is 3 liters.  We just do not know the tank volume.


{{{T(1/2-1/3)=3}}}
{{{T(1/6)=3}}}
{{{T=3*6}}}
{{{T=18}}}