Let the tank has total capacity of x liters
When ¹/₃ is full it means to say that it has ˣ/₃ litres of water
Now when 100 litres water was added it became half full
Note:- half full = ˣ/₂ litres and 100 litres water was added to it = ˣ/₃ +100 litres
Therefore the tank can hold 600 litres of water...