Marbles Probability question using Bayes Rule

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Marbles Probability question using Bayes Rule
I have this question from my revision booklet that I cant understand.
A bag initially contained 4 white marbles and 9 black marbles. A marble was drawn and replaced by two marbles of the opposite color. Then another marble was drawn and also replaced by two marbles of the opposite color. A third marble was then drawn.

Find the probability that the first marble drawn was white if:

a) The second marble drawn was white

b) The third marble drawn was white

c) Both the second and third marbles drawn were white

My Attempt:

For a I used Bayes rule for a and computed the various probabilities and found out that a is 2/11. I'm not at all sure about b and c.

Best Answer

Let $W_k$ be the event of drawing a white marble on the $k$-th draw, and $B_k$ the complementary event.

So your solution should look somewhat like:

(a) $$\begin{align}\mathsf P(W_1\mid W_2)&=\dfrac{\mathsf P(W_1)~\mathsf P(W_2\mid W_1)}{\mathsf P(W_1)~\mathsf P(W_2\mid W_1)+\mathsf P(B_1)~\mathsf P(W_2\mid B_1)}\\[1ex]&=\dfrac{\tfrac{4}{4+9}\tfrac{3}{3+11}}{\tfrac{4}{4+9}\tfrac{3}{3+11}+\tfrac{9}{4+9}\tfrac{6}{6+8}}&=\dfrac{4(4-1)}{4(4-1)+9(4+2)}\\[1ex]&=\dfrac{2}{11}&\checkmark\end{align}$$

The others use Bayes' Rule too. Hint: tackle (c) first.

(b) $$\begin{align}\mathsf P(W_1\mid W_3)&=\dfrac{\mathsf P(W_1,W_2,W_3)+\mathsf P(W_1,B_2,W_3)}{\mathsf P(W_1,W_2,W_3)+\mathsf P(W_1,B_2,W_3)+\mathsf P(B_1,W_2,W_3)+\mathsf P(B_1,B_2,W_3)}\\[2ex]&=\tfrac{\mathsf P(W_1)~\big(\mathsf P(W_2\mid W_1)~\mathsf P(W_3\mid W_1,W_2)+\mathsf P(B_2\mid W_1)~\mathsf P(W_3\mid W_1,B_2)\big)}{{\mathsf P(W_1)~\big(\mathsf P(W_2\mid W_1)~\mathsf P(W_3\mid W_1,W_2)} + {\mathsf P(B_2\mid W_1)~\mathsf P(W_3\mid W_1,B_2)\big)+\mathsf P(B_1)~\big(\mathsf P(W_2\mid B_1)~\mathsf P(W_3\mid B_1,W_2)+\mathsf P(B_2\mid B_1)~\mathsf P(W_3\mid B_1,B_2)\big)}}\end{align}$$

(c) $$\begin{align}\mathsf P(W_1\mid W_2,W_3)&=\dfrac{\mathsf P(W_1)~\mathsf P(W_2\mid W_1)~\mathsf P(W_3\mid W_1,W_2)}{\mathsf P(W_1)~\mathsf P(W_2\mid W_1)~\mathsf P(W_3\mid W_1,W_2)+\mathsf P(B_1)~\mathsf P(W_2\mid B_1)~\mathsf P(W_3\mid B_1,W_2)}\end{align}$$

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