Challenge 3: Reading a Range Reference — Solution Walkthrough

The range B2:D5 covers every cell from B2 (top-left corner) through
D5 (bottom-right corner), inclusive. Written out individually, that's
every combination of columns B, C, D with rows 2, 3, 4, 5:

  B2  C2  D2
  B3  C3  D3
  B4  C4  D4
  B5  C5  D5

That's 3 columns × 4 rows = 12 cells total.

=SUM(B2:D5) adds together the values currently stored in all 12 of
those cells and returns a single total. If any of those 12 cells are
empty or contain text, SUM silently treats them as contributing zero
to the total rather than raising an error — it only adds up whatever
numeric values it actually finds in the range.

Notes:
- The two addresses either side of the colon are always the OPPOSITE
  corners of the rectangle the range describes — the colon means
  "and everything in between," not just "these two specific cells."
- A single-column or single-row range works the same way with one
  dimension fixed, e.g. A1:A10 is 1 column × 10 rows = 10 cells,
  which is exactly the range used in this chapter's own SUM example.
- This rectangle-of-cells mental model is worth having fully solid
  before Chapter 4, where SUM, AVERAGE, COUNT, and nested functions
  all lean on exactly this same range concept.