Question

Difficulty: HardInterpreting Linear Relationships in Context

A municipal water treatment facility utilizes 3 identical backup filtration units to process stormwater runoff. The average volume of water, V(t)V(t), in thousands of gallons, remaining to be filtered per unit tt hours after the units are activated is modeled by the equation:

3V(t)+14.4t=1623V(t) + 14.4t = 162

where 0t100 \le t \le 10. Which of the following is the best interpretation of the number 14.414.4 in this context?

  1. A
    The initial total volume of water, in thousands of gallons, remaining to be filtered by all 3 units combined.
  2. B
    The rate, in thousands of gallons per hour, at which water is filtered by each individual unit, which is 4.8.
  3. The rate, in thousands of gallons per hour, at which water is filtered by all 3 units combined.Answer
  4. D
    The rate, in thousands of gallons per hour, at which water is filtered by each individual unit, which is 4.6.

Answer

The rate, in thousands of gallons per hour, at which water is filtered by all 3 units combined.
The correct answer is the option stating that 14.4 represents the rate, in thousands of gallons per hour, at which water is filtered by all 3 units combined. In the given equation 3V(t)+14.4t=1623V(t) + 14.4t = 162, we can isolate the total volume remaining, 3V(t)3V(t), as 3V(t)=16214.4t3V(t) = 162 - 14.4t. In this form, the rate of change of the total volume is 14.4-14.4, indicating that the combined remaining volume decreases by 14.4 thousand gallons each hour.

Step-by-Step Solution

1
Express the total volume of water remaining to be filtered in terms of the variable V(t)V(t).
Since V(t)V(t) is the average volume of water remaining per unit and there are 3 identical units, the total volume of water remaining to be filtered by all 3 units combined is 3V(t)3V(t).
This allows us to analyze the total filtration system rather than the average per unit.
2
Isolate the term representing the total volume remaining, 3V(t)3V(t), in the given equation.
3V(t)=16214.4t3V(t) = 162 - 14.4t
This puts the equation into a standard linear form y=mx+by = mx + b where y=3V(t)y = 3V(t) is the dependent variable (total volume remaining) and x=tx = t is the independent variable (time in hours).
3
Interpret the slope and the y-intercept of the isolated linear equation.
The constant term 162 is the initial total volume of water (in thousands of gallons) remaining at t=0t = 0. The coefficient of tt, which is 14.4-14.4, represents the change in the total volume remaining per hour.
In a linear model y=mx+by = mx + b, the slope mm represents the rate of change of the dependent variable per unit increase of the independent variable.
4
Determine the contextual meaning of the absolute value of the rate of change, 14.4.
A rate of change of 14.4-14.4 thousand gallons per hour means that the total volume of water remaining decreases by 14.4 thousand gallons each hour. Therefore, 14.4 is the rate, in thousands of gallons per hour, at which water is filtered by all 3 units combined.
The rate at which the remaining volume decreases is equal to the rate at which the system filters the water.

Key Concept

Interpreting coefficients and constants in a linear relationship within a real-world context, particularly when variables represent averages or totals.
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