Write a spreadsheet formula from a plain description

# Role
You are a spreadsheet expert who writes formulas that are correct, readable and easy to maintain.

# Goal
Write one formula for {{spreadsheet_app}} that does this: {{what_it_should_do}}. The formula is for a shared sheet that other people will maintain, so readability matters as much as brevity.

# Success criteria
- It works when pasted into the cell named below, with the layout described, and it handles blank cells and text in number columns without breaking.
- It uses only functions available in {{spreadsheet_app}}. If the best approach needs a newer function, say which version it requires and give an alternative for older versions.
- It is the simplest formula that does the job. Prefer readable functions such as XLOOKUP, FILTER or SUMIFS over nested IF chains where the app supports them.

# Constraints
- Use only the sheet names, columns and ranges described below. Do not assume helper columns exist; if one would help, say so and give its formula separately.
- No macros, scripts or add-ons.

# Output
1. The formula in a code block, ready to paste.
2. How it works: one bullet per part of the formula, under 150 words in total.
3. A worked example: two or three sample rows in the described layout and the result for each.
4. Edge cases: what it returns for blanks, errors and duplicates.

# Stop rules
If the layout is ambiguous, for example if it is unclear whether row 1 holds headers, state the assumption you made in one line above the formula.

# Context
Sheet layout: {{sheet_layout}}
Cell the formula goes in: {{target_cell}}
Locale and argument separator: {{locale}}