The concept of Double-Then Complete or Double Facts helps in building a strong foundation for addition and mental math. When using this strategy, we first double a number and then adjust to find the final sum. For example, to solve 6 + 7, we can double 6 (which is 12) and then add 1 more to make 13. This method works well for different digits, such as 4 + 5 (double 4 = 8, then add 1 to get 9) or 8 + 9 (double 8 = 16, then add 1 to make 17). Practicing double facts like 1 + 1 = 2, 2 + 2 = 4, 3