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The Two Errand Boys
Answer :

All that is necessary is to add the two distances at which they meet to twice their difference. 
Thus 720 + 400 + 640 = 1760 yards, or one mile, which is the distance required. 

Or, put another way, three times the first distance less the second distance will always give the answer, only the first distance should be more than two-thirds of the second.

Medieval Brain Teasers