Casework

Split the problem into a few situations that do not overlap and together cover everything, solve each one, then add. The two dangers are cases that overlap, which double-counts, and cases that leave a gap, which undercounts.

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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 ladder7 problems. Three in a row finishes it.0/3 in a row · 0/7 solved
Practice — 5-point ladder7 problems. Three in a row finishes it.0/3 in a row · 0/7 solved
Skill quizFinish every ladder that has problems first.Finish the ladders above first

Before and after

Needs first

CNT-01Systematic listing

Opens up

CNT-09Complementary counting CNT-13Counting shapes hidden inside a figure CNT-14Counting cubes in a stack, including hidden ones CNT-17The pigeonhole principle NUM-14Operation puzzles and cryptarithms

In official papers

No indexed official paper question has been tagged with CNT-04 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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