A function show the relationship between an independent variable and a dependent variable. The independent variable (input) does not depend on other variables while the dependent variable (output) depend on other variables.
In this question, the volume of the water is the dependent variable and the time taken is the independent variable. Let y represent the volume of the water and x represent the time taken. Therefore:
y = 0.75x
The domain of the function is the set of independent variable, it can be gotten by determining x when y is minimum and maximum. Hence:
