browser mindreading

Out of context: Reply #12

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  • kpl0

    think of it this way:

    12 -> 1+2 = 3
    13 -> 1+3 = 4
    14 -> 1+4 = 5

    every time you add one to one side, you also add one to the other, and get the same difference.

    19 -> 1+9 = 10
    20 -> 2+0 = 2

    29 -> 2+9 = 11
    30 -> 3+0 = 3

    when the tens column increases, it's like you add one to both sides still, but subtract 9 on the right side. in the end, it's still a multiple of 9.

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