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[SOLVED] Selecting a Red Ball from an Urn
An urn contains 3 red and 7 black balls. Players A and B withdraw balls from the urn consecutively until a red ball is selected. Find the probability that A selects the red ball. (A draws the first ball, then B and so on. There is no replacement of the balls drawn.)
Axioms and basic theorems of probability.
This is similar to the dice problem I had posted about previously https://www.physicsforums.com/showthread.php?t=198863". Let S be the sample space. S = {r, br, bbr, bbbr, bbbbr, bbbbbr, bbbbbbr, bbbbbbbr} where bbbr for example denotes that A drew a black ball, then B drew a black ball, then A drew a black ball, then B drew a red ball. The probability that A selects the red ball is P{r} + P{bbr} + P{bbbbr} + P{bbbbbbr} where P{r} = 3/10, P{bbr} = 7/10 * 6/9 * 3/8, P{bbbbr} = 7/10 * 6/9 * 5/8 * 4/7 * 3/5 and P{bbbbbbr} = 7/10 * 6/9 * 5/8 * 4/7 * 3/6 * 2/5 * 3/4. Is this correct?
Homework Statement
An urn contains 3 red and 7 black balls. Players A and B withdraw balls from the urn consecutively until a red ball is selected. Find the probability that A selects the red ball. (A draws the first ball, then B and so on. There is no replacement of the balls drawn.)
Homework Equations
Axioms and basic theorems of probability.
The Attempt at a Solution
This is similar to the dice problem I had posted about previously https://www.physicsforums.com/showthread.php?t=198863". Let S be the sample space. S = {r, br, bbr, bbbr, bbbbr, bbbbbr, bbbbbbr, bbbbbbbr} where bbbr for example denotes that A drew a black ball, then B drew a black ball, then A drew a black ball, then B drew a red ball. The probability that A selects the red ball is P{r} + P{bbr} + P{bbbbr} + P{bbbbbbr} where P{r} = 3/10, P{bbr} = 7/10 * 6/9 * 3/8, P{bbbbr} = 7/10 * 6/9 * 5/8 * 4/7 * 3/5 and P{bbbbbbr} = 7/10 * 6/9 * 5/8 * 4/7 * 3/6 * 2/5 * 3/4. Is this correct?
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