Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A science club has two urns. Urn I holds red balls and black balls. Urn II holds red balls and black balls. A member draws one ball from Urn I at random and puts it into Urn II without looking at its colour. Another member then draws one ball at random from Urn II, which now holds balls. The club wants three numbers for a display. First, find the probability that the ball drawn from Urn II is red. Second, find the probability that both the transferred ball and the ball drawn from Urn II are red. Third, find the probability that the ball drawn from Urn II is black. The colour of the transferred ball splits the problem into two cases that cover everything and cannot overlap. Give decimals to four places.
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 cases. The transferred ball is red with probability and black with probability .
Step 2. Urn II after the transfer. After a red transfer, the chance of drawing red from Urn II is . After a black transfer, it is .
Step 3. Both red. .
Step 4. The second ball is red. Add the other path: black then red is . So .
Step 5. The second ball is black. .
Answer: about , and .
Check. Without the transfer, the chance of red from Urn II would be . The transfer adds one ball that is red more often than Urn II's own balls, so the chance rises a little, to about .
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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.