To find the rate \( r \), we can use the formula:
\[ r = \frac{d}{t} \]
where \( d \) is the distance and \( t \) is the time.
Given:
- Distance \( d = 4 \) kilometers
- Time \( t = 50 \) minutes
Substituting the values in:
\[ r = \frac{4 , \text{kilometers}}{50 , \text{minutes}} = 0.08 , \text{kilometers per minute} \]
Therefore, the rate at which Sam walked is 0.08 kilometers per minute.