Answer: The statement is not reasonable.
Explanation:
First, 1 millimeter is equivalent to a (1/1000) meters = 0.001 meters
so if Carly says that she walks 300 millimeters, she is walking:
300*0.001 meters = 0.3 meters
This would mean that she lives at a distance of less than half a meter of the school, which has no sense, so the statement is not reasonable.