It is easier to simply add two one-digit numbers than to mess about with estimation!
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6
Nothing special about it. 489 +65 554
999,999 is 6 digits Adding 1 makes it 1,000,000 = a 7 digit number.
Add the last digit (units digit) to twice the previous digit (tens digit). If this sum is divisible by 4, so is the original number.
they all are digit numbers