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In a class of beginning second graders, children explained with these typical replies how they solved the number fact 8 + 7: “I know 7 + 7 is 14, and 1 more is 15”; “8 + 2 makes 10. But 7 has 5 more, so the answer is 15”; and “I just knew the answer was 15.” Teaching basic number facts like 8 + 7 has been a goal of elementary mathematics instruction for more than 100 years and continues to be important today.
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Isaacs et al. (1999) studied this question.
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