Unknown in Addition & Subtraction

Determine the unknown whole number in an addition or subtraction equation relating three whole numbers

Usually learned at ages 6–7

Builds on: Inverse: addition undoes subtraction (helps), What the equals sign means

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On the map from: Marble Skill Taxonomy (v1), © Generative Spark, Inc. (withmarble.com), text CC BY-SA 4.0, database ODbL 1.0: https://github.com/withmarbleapp/os-taxonomy