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Sum the individual digits:  6+7+6+4+5+6+0+n= 34+n

 

For any number divisible by 9 the sum of its digits is also divisible by 9

 

Therefore if we set n = 2 the sum will be 36, which is divisible by 9. No other single digit when added to 34 will give a number divisible by 9, so the missing digit is 2, and the overall number is 67645602

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