Final answer:
The assumption made in a multi-product break-even analysis is that the unit selling price will not change.
Step-by-step explanation:
When preparing a multi-product break-even analysis, the assumption is ordinarily made that the unit selling price will not change. The break-even analysis is a financial tool used to determine the sales volume needed to cover all costs and achieve neither a profit nor a loss. By assuming a constant selling price, the analysis can focus on determining the break-even point in terms of quantity sold.