What is the capacity of the tank?

Question

A rectangular tank is 3/4  full of water.
If you add another 320 cubic centimeters of water, the tank will be 7/8  full.

Answer 1

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.