Exercise 1: Remembering the Last Manually Selected Entry — Possible Solution
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Lines added to /etc/default/grub:

GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true

Command needed afterward:

sudo update-grub

Explanation:

Per the chapter's own description, GRUB_DEFAULT=saved (rather than a
fixed number like 0) tells GRUB to boot whichever entry was last
saved as the default, instead of always booting the same fixed
position in the menu. GRUB_SAVEDEFAULT=true is the companion setting
that actually makes GRUB save the entry a user manually selects at
the boot menu as the new "saved" default going forward -- without
this second line, GRUB_DEFAULT=saved would have nothing being
actively saved to reference. Both lines are needed together for the
"remember whichever entry was last chosen" behavior to actually work.
As with any change to /etc/default/grub, sudo update-grub is required
afterward to regenerate grub.cfg from the edited source file -- per
the chapter's own core lesson, the edit alone has no effect until
this command runs.

WHY THIS WORKS AS AN ANSWER
------------------------------
This provides both required settings (not just GRUB_DEFAULT=saved
alone, which wouldn't be sufficient by itself) and correctly includes
the update-grub step the chapter emphasizes is required for any
/etc/default/grub change to actually take effect.