Urgent!

[prime scaffold]

can someone please explain how to solve this

[vernal compass]

The first thing I would do is make a tree diagram showing the various possible sequences of events

like this!

Next, I'd find the probability of each potential chain of events, by multiplying the values down each path. For example, the probably that a person is infected and tests negative is .005*.15

Next, the probability that a person has the virus if they've tested positive would be the probability that they're positive and have the virus, divided by the overall probability that they tested positive

P(Infected | +) = P(Infected ^ +) / P(+)
And P(+) is simply P(Infected ^ +) + P(Healthy ^ +)

Part B is similar, just using the other values

P(Healthy | -) = P(Healthy^-)/P(-)
P(-) = P(Healthy^-) + P(Infected^-)