Suppose 80% of kids who visit a doctor have a fever, and 25% of kids with a fever have sore throats. What's the probability that a kid who goes to the doctor has a fever and a sore throat?
So what you do is find 25% of 80% of 100%. So, 8 out of 10 kids have a fever, and 2 out of 8 kids have a sore throat. So you would have 5 out of 10 kids have a sore throat, and a fever
