Given
A small tank can hold 150 liters and the difference is 300 liters. So, the large tank can hold 150 + 300 = 450 liters.
So,
volume in the large tank when it is half full = 450/2 = 225 liters
volume in the small tank when it is full = 150 liters
Difference = 225 - 150 = 75 liters
Total volume = 150 + 225 = 375 liters