Overcount, then correct

A deliberate strategy: count in a way that is easy but wrong, work out exactly how much it overcounts by, and divide or subtract. Naming the overcount precisely is the difficulty, not the arithmetic.

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Work through it

LearnWhat it is, how to spot it, a worked example, and where it goes wrong.
Practice — 3-point ladderNo problems written at this tier yet.Nothing written at this tier yet
Practice — 4-point ladder2 problems. Three in a row finishes it.0/3 in a row · 0/2 solved
Practice — 5-point ladder4 problems. Three in a row finishes it.0/3 in a row · 0/4 solved
Skill quizFinish every ladder that has problems first.Finish the ladders above first

Before and after

Needs first

CNT-10Inclusion–exclusion with two sets

Opens up

Nothing yet. No other skill in the Atlas lists this one as a prerequisite.

In official papers

No indexed official paper question has been tagged with CNT-16 yet. The index grows one paper at a time, and it records only question numbers, point values and our own skill tags — never the problems themselves.

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