∵ 1+2+3+…+20 12=(20 12+ 1)×20 12÷2,

The sum of the single digits of these 20 12 natural numbers is 8.

and

All other numbers have been erased, leaving only 12 and another number.

∴ Another number is the single digit of the erased sum of three numbers, which must be less than the single digit of the sum of 10 and 12, so it is 6.

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