>>74116373Classic off-by-one error. Very common math mistake, especially in computer science. Specifically, you made a "fencepost"-type off-by-one error.
https://en.wikipedia.org/wiki/Off-by-one_errorYou wouldn't count the 3 feet either in front of the first chumbud or the 3 feet behind the last chumbud, so the actual answer is 1,321 people for one mile or 9241 people for 7 miles.