Dice probability with limited reroll

[Dan K]

Hello,

I'm having difficulty understanding this problem and I was wondering if someone had an answer.

Let
[tex] n = \text{Number of dice} = 3[/tex]
[tex] p_s = \text{Single die success probability (when the face is 3 or more)} = \frac{1}{2}[/tex]
[tex] k = \text{Number of successful die} \in 0, 1, 2, 3[/tex]
[tex] P(n, k, p) = (^n_k) p^k(1-p)^{n-k}[/tex]

The probability of getting 2 success [tex] (k=2)[/tex] if each die is rolled only once [tex] (p = p_s = \frac{1}{2})[/tex] is
[tex] P(3,\ 2,\ \frac{1}{2}) = (^3_2)\left(\frac{1}{2}\right)^2\left(1-\frac{1}{2}\right)^{3-2} = \frac{3}{8} = 0.375[/tex]

If we have the option to reroll any failed die once,
then [tex] p = p_s + (1-p_s)p_s = \frac{3}{4}[/tex]
and [tex] P(3,\ 2,\ \frac{3}{4}) = (^3_2)\left(\frac{3}{4}\right)^2\left(1-\frac{3}{4}\right)^{3-2} = \frac{27}{64} = 0.421875[/tex]

What is the probability if we can reroll only one failed die (instead of all 3)?

 

[Dan K]

Out of the 3 dice, 2 [tex](n_{1,2} = 2)[/tex] are rolled once [tex](p_{1,2} = p_s = \frac{1}{2})[/tex] and 1 die [tex](n_3 = 1)[/tex] is rolled twice [tex](p_3 = p_s + (1 - p_s)p_s = \frac{3}{4})[/tex]

The probability of rolling 2 successes when all three dice are rolled and then one is rerolled is:
[tex] P(\text{Rolling 2 successes on the 1-roll dices})\times P(\text{Rolling 0 successes on the 2-roll die})[/tex]
+[tex] P(\text{Rolling 1 success on the 1-roll dices})\times P(\text{Rolling 1 success on the 2-roll die})[/tex]

[tex]= P\left(2,\ 2,\ \frac{1}{2}\right)\cdot P\left(1,\ 0,\ \frac{3}{4}\right)[/tex] + [tex] P\left(2,\ 1,\ \frac{1}{2}\right)\cdot P\left(1,\ 1,\ \frac{3}{4}\right)[/tex]

[tex]= (^2_2)\left(\frac{1}{2}\right)^2\left(1 - \frac{1}{2}\right)^{2 - 2}\cdot(^1_0)\left(\frac{3}{4}\right)^0\left(1 - \frac{3}{4}\right)^{1 - 0}[/tex]
+[tex] (^2_1)\left(\frac{1}{2}\right)^1\left(1 - \frac{1}{2}\right)^{2 - 1}\cdot(^1_1)\left(\frac{3}{4}\right)^1\left(1 - \frac{3}{4}\right)^{1 - 1}[/tex]
[tex]= 0.4375[/tex]

However, this seems wrong because it's greater than if I were to roll 3 dice with rerolling [tex] P\left(3, 2, \frac{3}{4}\right) = 0.421875[/tex]

What am I missing?

P.S. Sorry for the formatting. The tex on this site doesn't support \binom{x}{y} nor the '+' sign.

 

[Dason]

P.S. Sorry for the formatting. The tex on this site doesn't support \binom{x}{y} nor the '+' sign.

Check this out: http://www.talkstats.com/showthread.php/14986-How-to-use-Math-tags

Use math tags instead of tex tags and things will work better.