How can the concept of a composite unit be used to explain why an unfavorable total sales-mix

Question:

How can the concept of a composite unit be used to explain why an unfavorable total sales-mix variance of contribution margin occurs?

Contribution Margin
Contribution margin is an important element of cost volume profit analysis that managers carry out to assess the maximum number of units that are required to be at the breakeven point. Contribution margin is the profit before fixed cost and taxes...