You are a construction worker standing still on 12th Ave N in Fargo, and you watch a minivan drive past at 25 mi/hr eastward. A kid in the back throws a ball at his brother in the front seat at 7 mi/hr.
A. How fast does the ball move, as observed by you? (Positive indicates east, negative indicates west.)
B. How fast does the ball move, according to the kid who threw it?
C. if instead the kid in the front seat throws a rock at the kid in the back seat at 7 mi/hr, how fast would you observe the rock to travel?