It is know that is an 18-digit number, with nine of the ten digits from 0 to 9 each appearing twice. Find the sum of the digits of .
Honor Bonus : Do it without calculating the product.
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Every number is congruent mod 9 to its digit sum. We can compute the factors of N mod 9 by repeatedly iterating digit sums. So N ≡ 4 ⋅ 8 ⋅ 5 ⋅ 1 9 ⋅ 5 3 m o d 9 ≡ 4 ⋅ 8 ⋅ 5 ⋅ 1 ⋅ 8 m o d 9 ≡ 2 m o d 9 If k is the digit left out of N , then the digit sum of N is 9 0 − 2 k . This is 2 mod 9 , so we get k ≡ 8 mod 9 , so k = 8 and the digit sum of N is 7 4 .