Challenge 1: Brightening Only a Subject's Face — Solution Walkthrough

The approach:

Add a brightness-related Adjustment Layer (e.g. Levels or Curves)
above the photo, which by default brightens the entire image. Then
select that Adjustment Layer's own automatically-attached mask and
paint black over the mask everywhere except the face — per Chapter
5's own black-conceals-white-reveals rule, this restricts the
brightening effect to only the area left white (the face), while the
rest of the image reverts back to its original, unadjusted
appearance.

Why this works non-destructively:

Per this chapter's own material, the Adjustment Layer itself never
touches the actual pixel data underneath — it applies its effect live,
on top, exactly like a layer mask restricts a regular layer's own
visibility. The mask on the Adjustment Layer can be repainted,
adjusted, or removed entirely at any point afterward, and the original
photo's own pixel data remains completely unchanged throughout.

Why this specifically uses two chapters' worth of material together:

This exercise combines this chapter's own Adjustment Layer concept
with Chapter 5's own masking rule directly — exactly the connection
this chapter's own tip box draws explicitly, since an Adjustment
Layer's mask behaves identically to any other layer's mask.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise is a direct, practical application of this chapter's own
Adjustment-Layer-plus-mask combination to a concrete, realistic
photo-editing task.