Wikibooks: "Macroeconomics: Multiplier Process"
Read this article about the simple multiplier. You should be able to use the formula from this article to calculate the multiplier's numeric value given MPC = 0.80. The result suggests an outcome such as income or GDP increases about five times the amount of a change in consumer expenditures, as articulated in the readings.
Click http://en.wikibooks.org/wiki/Macroeconomics/Multiplier_Process link to open resource.