A tank is half filled and contains 15litres.what fraction of the tank is full if it can contain 18litres?
contradiction !
If your tank is half full and it contains 15 L, then the full capacity must be 30 L
If the tank only can hold 18 L as you said, then at 15 L it is not "half filled"
If you are asking what fraction 15 is of 18 ???
15/18 = 5/6