Challenge 1: A Live-Recalculating Formula Across Two Sessions — Solution Walkthrough

Setup: a Sheet shared with a second account, one person edits a raw
input cell while the other actively views a dependent formula
elsewhere.

What the second person sees:

The formula's displayed result updates automatically and live, the
moment the first person's edit registers — with no need to refresh
the page, click into the formula cell, or take any action at all.
The number simply changes on screen, in real time, reflecting the
new input value, exactly as if the second person had made the edit
themselves.

WHY THIS WORKS AS AN ANSWER
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This is the direct, hands-on confirmation of this chapter's own core
claim: the same real-time sync mechanism Chapter 2 described for
shared text applies identically to a calculation dependency chain —
a formula's own inputs changing anywhere in the live shared document
triggers an immediate, visible recalculation for every current
viewer, not just the person who made the edit.