Challenge 1: Building the Related Data Model and DAX Measures — Solution Walkthrough

Setup:
  1. A small Products table (ProductID, Category, Subcategory) — 5-6
     rows, ProductID unique.
  2. Two or three small Sales CSVs, each with ProductID, Date, Amount,
     Cost, imported via Get Data > From Folder (Chapter 4) and
     combined into one Sales query.
  3. Both loaded into the Data Model (tick "Add this data to the Data
     Model" when creating a PivotTable, or via Power Query's Close &
     Load To dialog).
  4. Power Pivot > Manage > Diagram View: drag Sales[ProductID] onto
     Products[ProductID] to create the relationship.
  5. Power Pivot > Measures > New Measure, twice:

     Total Sales := SUM(Sales[Amount])

     Profit Margin := DIVIDE(SUM(Sales[Amount]) - SUM(Sales[Cost]), SUM(Sales[Amount]))

  6. Insert > PivotTable > "Use this workbook's Data Model." Drag
     Category (from Products) into Rows, and both Total Sales and
     Profit Margin (from the Measures list) into Values.
  7. Insert > Slicer, tick Category.

Confirming correctness:

Click a single category button on the Slicer. Total Sales should
update to show only that category's total (a plain SUM restricted by
the active filter), and Profit Margin should recalculate as
(that category's Sales total - that category's Cost total) / that
category's Sales total — NOT simply pick out one row's own margin,
since DAX measures always compute on the currently-filtered totals,
exactly as Chapter 5 described.

WHY THIS WORKS AS AN ANSWER
------------------------------
Both measures are defined once, in the Data Model itself, rather than
inside any one specific PivotTable — which is precisely why they
continue to work correctly the moment the Slicer changes what's
currently filtered. A Chapter 3-style calculated field, by contrast,
could only ever have been built inside a single PivotTable using a
single table's own fields; it could never reach across the
relationship to combine Products' Category grouping with Sales'
Amount and Cost totals the way these two measures do.