If fair dodecahedron is rolled until at least $k$($k$ is fixed between 2 and 12) is gotten, and $X$ is the sum of all numbers appeared until the last time, what is $E(X)$?
Find the expected value of a dice sum
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0An individual roll is at least $k$. – 2012-04-15
2 Answers
The probability that any roll is greater than or equal to $k$ is $ \frac{13-k}{12} $ so the expected number of rolls until a roll of $k$ or greater is $ \frac{12}{13-k}. $ All but the last one of these rolls is less than $k$, so the sum of those rolls has an expected value of $ \left( \frac{12}{13-k} -1 \right) \frac{1+ (k-1)}{2}. $ Add to this the expected value of the final roll $ \frac{k+12}{2} $ and so the expectation of the sum is $ \left( \frac{12}{13-k} -1 \right) \frac{1+ (k-1)}{2} + \frac{k+12}{2} = \frac{78}{13-k}. $
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0And now I've edited my answer since Didier is now answering the right question. – 2012-04-13
Let $n=12$ denote the number of faces. If the first roll is $i\geqslant k$, $X=i$. If the first roll is $i\lt k$, $X=i+X'$ where $X'$ is distributed like $X$. Hence, $ \mathrm E(X)=\frac1n\sum_{i\geqslant k}i+\frac1n\sum_{i\lt k}\left(i+\mathrm E(X)\right)=\frac1n\sum_{i=1}^ni+\frac1n\mathrm E(X)\sum_{i=1}^{k-1}1, $ that is, $ n\mathrm E(X)=\frac{n(n+1)}2+(k-1)\mathrm E(X), $ hence $ \mathrm E(X)=\frac{n(n+1)}{2(n-k+1)}=\frac{78}{13-k}. $
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0Doesn't it depend on $k$? – 2012-04-13