<s>[INST] Probability that I will get a $4$ consecutive numbers I have five dice to roll. I roll them. What is the probability that I will get a straight with exactly four consecutive numbers and not $5$?
There are three options: $1,2,3,4$ or $2,3,4,5$ or $3,4,5,6$.
I have $1,2,3,4,*$ where $*$ can be either $1/2/3/4/6$. It cannot be five. Now we do $4!*5$.
Next, I take $2,3,4,5,*$ where $*= 2,3,4,5$. We get $4!*4$ and $4!*5$ for $3/4/5/6$.
I get the probability $(4!*14)/6^5$ but there are some complexities involved. Can anyone explain this clearly? I tried $4!*5*5$ since the position of * is variable. Now, I get $(4!*70)/6^5$ which is not correct either.
Can someone explain it systematically?
 [/INST] Imagine tossing the dice one at a time, and recording the results, or equivalently labelling the dice A to E, and recording the results as a string of length $5$, result on A, result on B, and so on. There are $6^5$ possibilities, all equally likely. Now we count the favourables. 
The "straight" part can be of any of type $1,2,3,4$, $2,3,4,5$, or $3,4,5,6$.
First we deal with the $2,3,4,5$. In order not to get $5$ in a row, we must avoid $1$ and $6$, so we must double up something. What we double up can be chosen in $4$ ways. For each of these ways, the smallest non-doubled number can be placed in $5$ ways, then the second smallest in $4$ ways, then the third smallest in $3$ ways. Now the doubleton falls in the remaining spaces.
That gives a total of $(4)(5)(4)(3)=240$ ways to have  $2,3,4,5$ as the "straight" part.
Now we deal with $1,2,3,4$. We could have the other number be a $6$, and then the $5$ numbers can be arranged in $5!=120$ ways. 
Or else we could double up one of our numbers. We have already analyzed this, and seen there are $240$ ways to do it. So there are $360$ patterns where the straight part is $1,2,3,4$.
Similarly, there are $360$ patterns where the straight part is $3,4,5,6$.
So our total is $240+360+360=960$, and the required probability is
$$\dfrac{960}{6^5}.$$   
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