What foundational skills should I focus on to prepare students for success in algebra?

Students need to understand the foundational principles of algebra that include properties for each of the operations and the idea of substitution in variables. Order of operations is also a pivotal skill for Algebra. The key to understanding is knowing that algebra involves equivalencies. Because this can often be very abstract for most upper elementary students, providing as much context as possible makes it easier. I HIGHLY recommend https://borenson.com/product/the-hands-on-equations-learning-system/

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