A rectangular fish tank having dimensions of 100 cm by 40 cm by 60 cm is half full of water. How many litres of water are there in the tank?

The answer to this is pretty straightforward. First, the volume a tank measuring 100 by 40 by 60 cm is 100x60x40cm = 240,000 cm^3 or cubic cm. Since one liter = 1000 cm^3, the total tank volume is thus 240 liters. If the tank is half full, then simply halve the amount of water so the tank would contain 120 liters if half full. The same approach can be used for any tank or container assuming it's volume is known, or can be calculated.