Worked example
A simplified reservoir contains 100 units. During one year, inputs are 12 and outputs 9 units. What is its ending stock?
Answer & why
Answer: 103 units
100 + 12 − 9 = 103 units.
Grade 12 · Science
Track a simplified environmental stock
Use a stated stock-and-flow model to calculate change while distinguishing model assumptions from real-world forecasts.
6 authored questions + worked example
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All questions
All 6 authored questions for this skill.
Question 1
A model starts with 50 units and assumes a constant net addition of 4 units each year. What stock does it project after three years?
- 62 units
- 12 units
- 54 units
- 150 units
A hint
Ending stock = starting stock + inputs − outputs; keep all quantities in the same units and time interval.
Answer & why
Answer: 62 units
50 + 3 × 4 = 62 units under the stated constant-flow assumption.
Question 2
A model predicts constant outflow, but observations show outflow rises as the stock grows. What should be reconsidered?
- Whether observations can ever inform a model.
- Whether changing evidence must be ignored.
- Only the model diagram's color.
- The constant-outflow assumption.
A hint
Ending stock = starting stock + inputs − outputs; keep all quantities in the same units and time interval.
Answer & why
Answer: The constant-outflow assumption.
The observations challenge a specific assumption used in the projection.
Question 3
A stock stays constant while 20 units enter and 20 leave each year. What description fits?
- A steady stock with ongoing flows.
- Inputs exist but outputs are zero.
- No material moves.
- The stock must grow by 40 each year.
A hint
Ending stock = starting stock + inputs − outputs; keep all quantities in the same units and time interval.
Answer & why
Answer: A steady stock with ongoing flows.
A constant total can coexist with balanced movement across the boundary.
Question 4
A model includes only one reservoir and fixed flows. Which claim exceeds its stated scope?
- Additional reservoirs could alter the model.
- It captures every feedback in the real environmental system.
- It can calculate its own assumed stock changes.
- Its predictions depend on its assumptions.
A hint
Ending stock = starting stock + inputs − outputs; keep all quantities in the same units and time interval.
Answer & why
Answer: It captures every feedback in the real environmental system.
A one-reservoir fixed-flow model omits processes and feedbacks present in larger systems.
Question 5
A simplified reservoir contains 100 units. During one year, inputs are 12 and outputs 9 units. What is its ending stock?
- 3 units
- 103 units
- 121 units
- 97 units
A hint
Ending stock = starting stock + inputs − outputs; keep all quantities in the same units and time interval.
Answer & why
Answer: 103 units
100 + 12 − 9 = 103 units.
Question 6
In a simplified carbon reservoir, annual inputs and outputs are both 8 units. What is the modeled annual stock change?
- −16 units
- 16 units
- 0 units
- 8 units
A hint
Ending stock = starting stock + inputs − outputs; keep all quantities in the same units and time interval.
Answer & why
Answer: 0 units
Equal inflow and outflow give zero net change, not zero movement.