From guess and check to algebra
The problem:
Guess a number of . The are whatever is left over. Add a few guesses and watch the last column.
Each guess is a point: across, total up. Plot several, then draw the line.
5
Pick a guess. The arithmetic you repeat each time is the same as the algebra, with a letter where the guess was.
7
With a guess
With a letter
As a system of two equations
Let = and = . Two facts, two equations.
Drag to guess. Your guesses leave points on the graph. When you are ready, reveal the algebra.
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