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Say It in Letters

Name the unknown with a letter and build the quantity forwards as an expression, then let like terms collect. A two-digit number becomes 10a + b, a border path becomes strips of x, a sum pyramid becomes weighted sums of its base. Once written, the expression explains why a trick always works or why a total never depends on the letter -- by simplifying it, not by undoing the steps.

Builds on: Place-Value RegroupingWork Backwards to Recover a Start Value

Progression (4)