To express the distance \( d \) in kilometers as a function of time \( t \) in hours, we can use the formula for distance:
\[ d = \text{speed} \times \text{time} \]
In this case, the pedestrian's speed is 3 km/hr. Therefore, we can write:
\[ d(t) = 3t \]
where \( d(t) \) is the distance in kilometers and \( t \) is the time in hours. This equation shows that the distance traveled by the pedestrian is directly proportional to the time they walk.