Sum of digits

N = 13 × 17 × 41 × 829 × 56659712633 \large N = 13 \times 17 \times 41 \times 829 \times 56659712633 It is know that N N 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 N N .

Honor Bonus : Do it without calculating the product.

This problem is taken from the 7th Hong Kong Pui Ching Invitational Mathematics Competition, year 2008.


The answer is 74.

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1 solution

Patrick Corn
Jul 21, 2015

Every number is congruent mod 9 9 to its digit sum. We can compute the factors of N N mod 9 9 by repeatedly iterating digit sums. So N 4 8 5 19 53 m o d 9 4 8 5 1 8 m o d 9 2 m o d 9 \begin{aligned} N &\equiv 4 \cdot 8 \cdot 5 \cdot 19 \cdot 53 \mod 9 \\ &\equiv 4 \cdot 8 \cdot 5 \cdot 1 \cdot 8 \mod 9 \\ &\equiv 2 \mod 9 \end{aligned} If k k is the digit left out of N N , then the digit sum of N N is 90 2 k 90-2k . This is 2 2 mod 9 9 , so we get k 8 k \equiv 8 mod 9 9 , so k = 8 k = 8 and the digit sum of N N is 74 \fbox{74} .

sir, i didn't understand how you solved this question. could you please explain your solution or refer to a site or book where i can clear my doubts on the method. I'm having doubts from the first sentence itself. thanks a lot!

Lipsa Kar - 5 years, 10 months ago

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