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Encourage questions like "what could come before . . . . . and what before that" ?
Working backwards from the goal is a mathematical thinking skill.
Especially encourage explanation - accounting for pattern.
There is lots of interest in this simple mechanical process - look out for the triangle numbers!
Above all, if possible, let each insight prompt fresh questions to consider, each new question will illuminate more of the structure.
15 = 7 + 8 and 10 = 1 + 2 + 3 + 4. Can you say which numbers can be expressed as the sum of two or more consecutive integers?
I have forgotten the number of the combination of the lock on my briefcase. I did have a method for remembering it...