Question 980173
The moving army is the exact opposite of the
current in a stream because the motion of the
army has ZERO effect on the messenger's
motion.
If you are moving in the same direction as the army
at 10 mi/hr and the army is moving at 1 mi/hr
you can imagine the army is stopped and you are
moving at {{{ 10 - 1 = 9 }}} mi/hr
If the army was a stream moving in the same
direction, the current would ADD to the
messenger's motion
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If you are moving opposite to the direction of the army,
imagine that the army is stopped, and you are moving
at the sum = {{{ 10 + 1 }}} mi/hr
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Just think about the current of a stream having a direct
effect on a boat ( adding or subtracting ) 
Think about the army as the background against which 
you are moving either whizzing  by you making it easy
to get where you are going, or the background is moving
away from you making it harder to get where you're going
Hope this helps