Question 149458
The "boy" is moving in only two directions -- north and south.
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It might be more understandable if you replace "up" for "north" and "down" for "south":
In Paris, a boy walks 4 km up, 5 km down, 2 km up, and 3 km down. How
far is the boy from his starting point?
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Then if you think of "up" as + and "down" as - we get:
4 - 5 + 2 - 3 
= -1 + 2 - 3
= 1 - 3  
= -2 
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Remember, the "negative" translate to south
The boy is 2 km south from his starting point.