A set of $n$ standard dice are shaken and randomly placed in a straight line. If $n < 2r$ and $r < s$, then the probability that there will be a string of at least $r$, but not more than $s$, consecutive $1$'s can be written as $\frac{P}{6^{s+2}}$. Find an explicit expression for $P$.
To find an explicit expression for \( P \), we need to consider the probability that there will be a string of at least \( r \), but not more than \( s \), consecutive 1's in a sequence of \( n \) dice rolls. 

1. **Understanding the Problem**:
   - We have \( n \) dice rolls.
   - We are looking for a string of at least \( r \) but not more than \( s \) consecutive 1's.
   - The probability can be written as \( \frac{P}{6^{s+2}} \).

2. **Probability Calculation**:
   - Each die has 6 faces, so the total number of possible outcomes for \( n \) dice is \( 6^n \).
   - We need to count the number of favorable outcomes where there is a string of at least \( r \) but not more than \( s \) consecutive 1's.

3. **Counting Favorable Outcomes**:
   - We need to count the number of ways to place a string of exactly \( k \) consecutive 1's (where \( r \leq k \leq s \)) in \( n \) dice rolls.
   - For each \( k \) (where \( r \leq k \leq s \)), the number of ways to choose \( k \) positions out of \( n \) is \( \binom{n}{k} \).
   - The remaining \( n - k \) positions can be any of the 6 faces, so there are \( 6^{n-k} \) ways to fill these positions.

4. **Summing Over All Valid \( k \)**:
   - We sum over all possible values of \( k \) from \( r \) to \( s \):
   \[
   P = \sum_{k=r}^{s} \binom{n}{k} 6^{n-k}
   \]

5. **Simplifying the Expression**:
   - The term \( 6^{n-k} \) can be factored out of the sum:
   \[
   P = 6^n \sum_{k=r}^{s} \binom{n}{k} 6^{-k}
   \]

6. **Final Expression**:
   - The probability that there will be a string of at least \( r \), but not more than \( s \), consecutive 1's is:
   \[
   \frac{P}{6^{s+2}} = \frac{6^n \sum_{k=r}^{s} \binom{n}{k} 6^{-k}}{6^{s+2}}
   \]
   - Simplifying further:
   \[
   P = 6^{s+2} \sum_{k=r}^{s} \binom{n}{k} 6^{n-k}
   \]

The final answer is \( \boxed{ P = 6^{s+2} \sum_{k=r}^{s} \binom{n}{k} 6^{n-k} } \).