Challenge 1: Explain the Odd-Number Rule — Possible Solution
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An election requires a MAJORITY of voting members to agree on a new
primary — more than half. With an EVEN number of members, a network
partition can split the set into two equal-sized halves that can each
see their own members but not the other half.

With 4 members split 2-and-2, NEITHER half has a majority (2 out of 4
is exactly half, not more than half) — so neither half can elect a
primary, and the whole replica set becomes unable to accept writes
until the partition heals. This is worse than it sounds: adding a 4th
member for "extra redundancy" can actually make the set MORE fragile
against this specific failure mode, not less.

With an ODD number like 3 or 5, any even split still leaves one side
with a genuine majority. A 3-member set split 2-and-1 still has one
side with 2 out of 3 — a real majority — so that side can always elect
a primary and keep accepting writes, while the minority side correctly
steps back and refuses to elect one of its own (avoiding a dangerous
"two primaries at once" split-brain scenario).

So the fix for "we want more redundancy" isn't just "add one more
member" — it's specifically add ONE MORE to reach the next odd number
(3 to 5, not 3 to 4), preserving the guarantee that any partition
still produces a clear majority on one side.