Two-Category Counts from a Total
Two interacting categories (right vs wrong answers, animals with different leg counts, coins of different values) combine into one total; the counts are recovered by assuming one case and adjusting, or by systematic guess-and-check. This is the elementary precursor to systems of equations.
grade 2–3 Measurement & dataOperations Guess and CheckMake a Systematic List
Progression (3)
g2 1. Count ways to make an amount with coins Measurement & data
g3 2. Guess-and-check a legs problem Operations
g3 3. Find right and wrong counts from total score Operations