Understanding Addition and Subtraction

Your class is working on subtracting 4 from 1000 and Toby does the following. Can you tell what he was thinking? Do you know how to help him?

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Thumbnail: The abacus is a calculating tool that has been in use since ancient times and is still in use today. The abacus consists of a number of rows of movable beads or other objects, which represent digits. One of two numbers is set up, and the beads are manipulated to implement an operation involving a second number (e.g., addition), or rarely a square or cubic root. (Unsplash Lisense; Crissy Jarvis via Unspash)