Exercise 2: What Happens to a Party's List Seats When Its District Seats Outpace Its Vote Share — Possible Solution
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The chapter's own real, verified compensatory mechanism explains this
directly: in Germany's real Bundestag system, the total number of
seats each party holds - including its own district seats, not just
its list seats - is calculated proportionally against that party's
own real party-list vote share, not against the district results
alone.

In this scenario, the party has won 30 real district seats outright
through FPTP, but its 20% share of the national party-list vote would,
under pure proportional allocation, normally entitle it to fewer than
30 total seats in the assembly. Because the real compensatory
mechanism works by calculating the party's own overall, proportional
seat entitlement first and then working out how many additional list
seats (if any) it should receive on top of its district seats, this
party's own real list-seat allocation would be reduced accordingly -
potentially down to zero additional list seats, since it has already
won more seats through districts alone than its real proportional
vote share alone would justify.

This is precisely the real, rebalancing function the chapter describes:
a party that overperforms in district seats relative to its own
national list-vote share receives fewer (or no) compensating list
seats, while a party that underperforms in districts relative to its
own list-vote share receives additional list seats to bring its total
representation back in line with its real, actual national vote share.
The system's own real point is to keep the overall assembly composition
proportional to the list vote, even though individual district results
can and do deviate from that proportional outcome on their own.

ANSWER: This party's own real list-seat allocation would be reduced,
likely to zero, because Germany's compensatory mechanism calculates
each party's total seat entitlement based on its national list-vote
share (20% here) rather than its district-seat count alone - since the
party has already won 30 district seats, more than its 20% list-vote
share alone would justify, it receives no further list seats to
rebalance the overall proportional outcome.

WHY THIS WORKS AS AN ANSWER
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This correctly applies the chapter's own real compensatory mechanism
to a specific numeric scenario, explaining the concrete consequence
(reduced or zero list seats) rather than just restating that the
system is "proportional" in the abstract.