Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A hospital store buys syringes from three suppliers. Supplier A provides of the syringes, supplier B provides and supplier C provides . The shares of faulty syringes are for A, for B and for C. A syringe is chosen at random from the store. Find the probability that it is faulty, the probability that it came from B and is faulty, and the probability that it is not faulty. Give decimals to four places.
For a partition , the law of total probability is . Multiply along every path that ends in , then add the paths. The cases must not overlap and must cover everything.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The paths. A and faulty: . B and faulty: . C and faulty: .
Step 2. Faulty. .
Step 3. From B and faulty. This is one path, .
Step 4. Not faulty. .
Answer: , and .
Check. The overall rate lies between and .
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