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3. Common Core State Standards Initiative. (2010). Common core state standards for mathematics. http://www.corestandards.org/wp-content/uploads/Math_Standards1.pdf/
4. Cowan, R. (2003). Does it all add up? Changes in children’s knowledge of addition combinations, strategies and principles. In A. J. Baroody & A. Dowker (Eds.), The development of arithmetic concepts and skills: Constructing adaptive expertise (pp. 35–74). Lawrence Erlbaum Associates.
5. Department for Education. (2013). The National curriculum in England: Key stages 1 and 2 framework document. Government of the United Kingdom. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/425601/PRIMARY_national_curriculum.pdf