My Math Forum Probability problem

 February 8th, 2018, 10:51 PM #2 Senior Member     Joined: Sep 2015 From: USA Posts: 2,431 Thanks: 1315 this is just an exercise in chasing down a path in a probability tree. $P[p_1\text{ gets drug A with + results.}] = \left(\dfrac 1 2\right)\left(\dfrac 3 5\right) = \dfrac {3}{10}$ Now we add a red ball to the bucket due to the success so that there are now 2 red balls and 1 green ball. $P[p_2 \text{ gets a placebo.}] = \dfrac 1 3$ We do nothing due to $p_2$ receiving a placebo so $P[p_3 \text{ gets drug A.}] = \dfrac 2 3$ We multiply all these probabilities together to get the probability of the chain so $P[p_1 =A^+,~p_2 = P,~p_3=A] = \dfrac{3}{10}\dfrac{1}{3}\dfrac{2}{3} = \dfrac{6}{90} = \dfrac{1}{15}$ Thanks from Shanonhaliwell

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