Challenge 1: SUM, AVERAGE, COUNT, and COUNTA on Mixed Data — Solution Walkthrough

Data: A1=10, A2="N/A", A3=20, A4=(blank), A5=30, A6="N/A"

=SUM(A1:A6)    → 60     (10 + 20 + 30 — the two "N/A" text entries
                          and the blank cell are simply skipped)
=AVERAGE(A1:A6) → 20    (60 divided by 3 — see explanation below)
=COUNT(A1:A6)   → 3     (only A1, A3, A5 actually contain numbers)
=COUNTA(A1:A6)  → 5     (every non-empty cell: A1, A2, A3, A5, A6 —
                          A4 is the only genuinely empty cell)

Why AVERAGE isn't SUM divided by 6:

AVERAGE divides the sum by the COUNT of numeric values it actually
found, not by the total number of cells in the range. There are 6
cells in A1:A6, but only 3 of them (A1, A3, A5) contain numbers — the
two "N/A" entries and the one blank cell are excluded from both the
sum and the count entirely, as if they weren't part of the range at
all. So AVERAGE calculates 60 / 3 = 20, not 60 / 6 = 10.

Notes:
- This is exactly the distinction this chapter's own AVERAGE section
  warns about: a blank cell and a cell containing the literal number
  0 are NOT treated the same way. If A4 had actually contained 0
  instead of being blank, COUNT would return 4 and AVERAGE would
  calculate 60 / 4 = 15 instead of 20.
- COUNT vs. COUNTA is the clearest possible illustration of "counts
  numbers" vs. "counts anything non-empty" — COUNT skips the text
  entries entirely, COUNTA counts every one of them.