[Math] Probability of rolling 3 and 4 in a row with 4 6-sided dice

probability

So I was playing a game and was wondering what the probability was to roll numbers in a row.

Four fair six-sided dice are rolled. What is the probability that three of the numbers will be in a row. Also, that all 4 of them will be in a row.

I've tried solving it but I couldn't get it. How can I solve this? I know that with 4 dice there are 1296 combinations but I have not come up with a way to determine the outcome.

Best Answer

For four in a row, there are three possible lowest numbers. There are $4!$ ways to roll the required set of four numbers for each one, so the chance is $\frac {3 \cdot 4!}{6^4}=\frac {72}{1296}=\frac 1{18}$

Three in a row is harder. There are four possible lowest numbers of the run. Presumably we are prohibited from having a run of four. The fourth die can then match one of the three we have. There are three ways to choose which one, then $\frac {4!}2$ ways to order the throw for $4 \cdot 3 \cdot 12=144$ rolls. We can also have four distinct numbers with three in a row. Three are six rolls that satisfy this, $1235, 1236,2346,1345, 1456,2456$. These six possibilities can be arranged in $4!=24$ ways for a total of $144$. The total probability is then $\frac {144+144}{1296}=\frac 29$

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