The tank needs 5 more liters of water to be filled.
Subtraction is one of the basic operations of mathematics where we remove numbers or expressions or objects away from a given value.
Amount of water needed to fill the tank = 8 liters
Amount of water Alejandro filled = 2 liters
Amount of water Anthony filled = 1 less than Alejandro = 2 - 1 = 1 liter.
Total amount of water filled = 2 liters + 1 liter = 3 liters
Remaining space can be filled with 8 liter - 3 liter = 5 liters.
Hence tank still need 5 liters of water to be filled.
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