If the capacity is c cubic centimetres, then ¾c+320=⅞c.
So, since ¾ is 6/8 then 320=c(7/8-6/8)=⅛c. Multiply through by 8: 2560=c. So the capacity is 2,560 cubic centimetres. 320 cubic centimetres is the difference between the fractions ⅞ and ¾, one eighth the capacity of the tank.