Let $r_1$, $r_2$, $\ldots$, $r_m$ be a given set of $m$ positive rational numbers such that $\sum_{k=1}^m r_k = 1$.  Define the function $f$ by $f(n)= n-\sum_{k=1}^m \: [r_k n]$ for each positive integer $n$.  Determine the minimum and maximum values of $f(n)$.  Here ${\ [ x ]}$ denotes the greatest integer less than or equal to $x$.
1. **Understanding the function \( f(n) \):**
   The function \( f(n) \) is defined as:
   \[
   f(n) = n - \sum_{k=1}^m \lfloor r_k n \rfloor
   \]
   where \( \lfloor x \rfloor \) denotes the greatest integer less than or equal to \( x \).

2. **Analyzing the sum \( \sum_{k=1}^m \lfloor r_k n \rfloor \):**
   Since \( r_k \) are positive rational numbers and \( \sum_{k=1}^m r_k = 1 \), we can write:
   \[
   \sum_{k=1}^m r_k n = n
   \]
   However, \( \lfloor r_k n \rfloor \leq r_k n \), so:
   \[
   \sum_{k=1}^m \lfloor r_k n \rfloor \leq \sum_{k=1}^m r_k n = n
   \]
   This implies:
   \[
   n - \sum_{k=1}^m \lfloor r_k n \rfloor \geq 0
   \]

3. **Finding the minimum value of \( f(n) \):**
   Since \( \lfloor r_k n \rfloor \) is the greatest integer less than or equal to \( r_k n \), the difference \( r_k n - \lfloor r_k n \rfloor \) is a fractional part, which lies in the interval \([0, 1)\). Therefore:
   \[
   0 \leq r_k n - \lfloor r_k n \rfloor < 1
   \]
   Summing over all \( k \):
   \[
   0 \leq \sum_{k=1}^m (r_k n - \lfloor r_k n \rfloor) < m
   \]
   Since \( \sum_{k=1}^m r_k n = n \), we have:
   \[
   n - \sum_{k=1}^m \lfloor r_k n \rfloor = \sum_{k=1}^m (r_k n - \lfloor r_k n \rfloor)
   \]
   Therefore:
   \[
   0 \leq f(n) < m
   \]
   Since \( f(n) \) is an integer, the minimum value of \( f(n) \) is \( 0 \).

4. **Finding the maximum value of \( f(n) \):**
   To find the maximum value, we need to consider the maximum possible value of the sum of the fractional parts \( r_k n - \lfloor r_k n \rfloor \). Since each \( r_k n - \lfloor r_k n \rfloor \) is less than 1, the maximum value of \( f(n) \) is achieved when the sum of the fractional parts is maximized. The sum of the fractional parts can be as close to \( m \) as possible but not equal to \( m \). Therefore, the maximum value of \( f(n) \) is \( m-1 \).

Conclusion:
The minimum value of \( f(n) \) is \( 0 \) and the maximum value of \( f(n) \) is \( m-1 \).

The final answer is \( \boxed{0 \text{ and } m-1} \).