Probability of Defective Items

[justin_pscl]

Hello, I needed some help with a problem. Okay, here it goes.

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 its model II. If a model is selected at random, find the probability that it will be defective.

Now there are two models. What I did was I added the 80% and 20% and I got 100%. The next thing I did was I added the 10% and the 18% to get 28%. After that I kind of lost myself. Looking forward to answer. Oh and the answer in the back of the back says its 0.116. Thanks.

[maggiemagnet]

What is the probability that the item you've picked is Model 1? Assuming that you've picked a Model 1, what is the probability that it is defective? Then what is the probability that you've picked a defective unit of Model 1?

What is the probability that the item you've picked is Model 2? Assuming that you've picked a Model 2, what is the probability that it is defective? Then what is the probability that you've picked a defective unit of Model 2?

Then add.