I was slow on this as well but now I get it, much like binary or hex, each number can only occupy one space so 12 is really 1+2 which is 3, then plus the 2 for a total of 5, plus the original 10 ( no ones, carry a 1 to the 10s place) so we 1*10, plus 1+2+2 which gives us the 15. Whew, no wonder I loved it when we got out of NEW MATH and my folks could help me.
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