A manufacturer makes two models of an item: model I, which accounts for 80% of unit sales, and model II which accounts for 20% of unit sales. Because of defects, the manufacturer has to replace (or exchange) 10% of its model I and 18% of model II. If a model is selected at random, find the probability that it will be defective.

Question

A manufacturer makes two models of an item:  model I, which accounts for 80% of unit sales, and model II which accounts for 20% of unit sales.  Because of defects, the manufacturer has to replace (or exchange) 10% of its model I and 18% of model II.  If a model is selected at random, find the probability that it will be defective.