Challenge 1: Locking a Fixed Discount Rate — Solution Walkthrough

Setup: D1 = 0.1 (10% discount rate), A2 = 20, A3 = 40, A4 = 60.

Formula typed into B2:
  =A2*$D$1

Filled down to B3 and B4, the result is:
  B3:  =A3*$D$1
  B4:  =A4*$D$1

Calculated values:
  B2 = 20 * 0.1 = 2
  B3 = 40 * 0.1 = 4
  B4 = 60 * 0.1 = 6

Explanation:

A2 is left as a plain relative reference because each row genuinely
needs its OWN price — that part of the formula should shift as it's
filled down, which is exactly what a relative reference does by
default.

$D$1 is fully absolute (both parts locked) because it points at the
single, shared discount rate that every row needs to multiply by —
that reference must NEVER shift, or rows below B2 would end up
multiplying by an empty cell (D2, D3...) instead of the actual rate,
exactly the silent-wrong-answer bug this chapter's warning box
describes.

Notes:
- Only $D$1 needed locking here — D1 isn't in a row or column that
  varies alongside the prices, so both its row and column need to
  stay fixed no matter where the formula is copied.
- If D1 ever moved to a different cell, only ONE formula (B2) and its
  fill-down copies would need re-checking, precisely because the
  absolute reference keeps pointing at "wherever D1 is" consistently
  across every row.