This model exemplifies the simple system of a bathtub. The total water volume in the bathtub (lWater) at any given time is determined by the water added from the faucet and the water removed through the drain. The volume of water added from the faucet is affected by the water inflow rate, and the volume of water draining out of the bathtub is affected by the outflow rate. In this model, the total water volume (lWater) increases over time because the inflow rate (0.5L/s) is greater than the outflow rate (0.2L/s).