The graph of the amount of water drained versus time is a linear graph.
We can determine the equation to be:

The domain and range of the function are :
Domain: Represents the set of allowable time(x values) that satisfies the equation:

Time cannot be negative.
Range : Represents the set of allowable Amount of water drained(y values):

The amount of water drained starts at 75 litres and stops at zero