Challenge 1: A Rectangle With an Auto-Implemented and a Computed Property — Possible Solution
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Program.cs:

class Rectangle
{
    public double Width { get; set; }
    public double Height { get; set; }
    public double Area => Width * Height;   // read-only, expression-bodied
}

var r = new Rectangle { Width = 4, Height = 5 };
Console.WriteLine(r.Area);

Output:

    20

Explanation:
Width and Height are auto-implemented properties -- one line each,
with the compiler generating the backing field automatically. Area
uses the expression-bodied get shorthand (=>) instead of { get; set;
}, computing its value fresh every time it's read rather than storing
one -- there's no backing field for Area at all, and no setter, since
it's derived entirely from the other two properties.

WHY THIS WORKS AS AN ANSWER
------------------------------
This uses auto-implemented properties for the two independent values
and a computed, read-only property for the derived one, matching the
chapter's own distinction between plain { get; set; } properties and
ones with real logic behind a still-field-like call site.