Exercise 1: app_port From group_vars vs. -e on the Command Line — Possible Solution
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The value that wins: app_port = 9090 (the -e value).

Explanation:

Following the chapter's own practical precedence order, inventory
variables (which is exactly what group_vars/webservers.yml is -- an
inventory-level variable file per ansible1-3) sit near the BOTTOM of
the list, above only role defaults. Extra vars, supplied via -e on the
command line, sit at the very TOP of the list, explicitly described in
the chapter as winning "always... no exceptions." Since -e is
specifically designed as the deliberate override mechanism for "just
this one run, use this value regardless of everything else defined
anywhere else," it doesn't matter that group_vars/webservers.yml also
sets app_port -- the command-line value of 9090 takes priority over
the inventory-level value of 8080 in every case, with no way for the
inventory-level setting to win instead.

WHY THIS WORKS AS AN ANSWER
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This correctly identifies -e as the winning value and justifies it by
placing both variable sources on the chapter's own precedence list
(inventory vars near the bottom, -e always at the top), rather than
just asserting which one wins without showing the reasoning.