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Documented Problem Solving: Calculating Equilibrium Output

This page is authored by Amber Casolari, Riverside Community College, based on an original activity by Amber Casolari.
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This material was originally created for Starting Point: Teaching Economics
and is replicated here as part of the SERC Pedagogic Service.


The Keynesian model of the economy was presented in class. The difference between equilibrium GDP and potential GDP was discussed. In this exercise, students are provided with data to calculate equilibrium GDP. In addition, students are asked to describe the process they used to calculate the equilibrium and analyze the state of the economy. In the second part of this assignment, students repeat the process using slightly different data.

Learning Goals

Students will:

Context for Use

This assignment is appropriate for any class size or institutional type. It is appropriate for an introductory or more advanced course in macroeconomics. Students should have a fundamental understanding of the Keynesian model.

Description and Teaching Materials

For this activity, a question similar to the one below is necessary. The numbers in the formula can be changed to create a new problem.

Mathematical Model of Equilibrium Output

Suppose the following information reflects the closed economy of Casolari Land. The Consumption Function is such that C=300+.75(DI), Investment is fixed at $450 and the Government has a balanced budget, where both purchases and taxes equal $1,000.

These equations describe the economy:

DI=Y-T [Disposable income is total income less taxes.]
C= 300 + .75(DI) [Consumption is determined by disposable income.)
E=C+I+G+NX [Aggregate demand is the total of consumption, investment, government purchases, and net exports.]
E=Y* [In equilibrium, total spending matches total income or total output.]

Calculate the equilibrium level of GDP for this economy (Y*).

Mathematical Model of Equilbrium Output (Microsoft Word 29kB Apr13 10)

Teaching Notes and Tips

Students must recognize that equilibrium GDP is not necessarily the same as potential GDP. They tend to believe that any equilibrium, per se, is good. In addition, they struggle with translating their understanding of the concepts of disposable income and a closed economy into a mathematical equation. Lastly, some students encounter problems solving simultaneous equations. Faculty should emphasize these concepts in class; having students work together or think-pair-share tends to alleviate these issues.


Students should be able to use the formulas given to find the equilibrium level of GDP and explain the thought process they used to do so, including any reference to notes, graphs, and definitions.

References and Resources

Angelo and Cross (1993)

Flavell (1970)