Maths Olympiad Prep

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Number theory Difficulty 5.0 AIME Prove it Soviet Union

Problem:
All two digit numbers from 1919 to 8080 inclusive are written down one after the other as a single number N=1920217980N = 192021\ldots 7980. Is NN divisible by 19801980?

Solution

Solution:
1980=22325111980 = 2^{2} 3^{2} 5 \cdot 11. NN is obviously divisible by 222^{2} and 55. The digits in odd position are 9+(0+1+2++9)+(0+1+2++9)++(0+1+2++9)+0=9+645=2799 + (0 + 1 + 2 + \ldots + 9) + (0 + 1 + 2 + \ldots + 9) + \ldots + (0 + 1 + 2 + \ldots + 9) + 0 = 9 + 6 \cdot 45 = 279. The digits in even position are 1+(2+2++2)+(3+3++3)++(7++7)+8=9+10(2+3++7)=2791 + (2 + 2 + \ldots + 2) + (3 + 3 + \ldots + 3) + \ldots + (7 + \ldots + 7) + 8 = 9 + 10(2 + 3 + \ldots + 7) = 279. So the sum of the digits of NN is 22792 \cdot 279 which is divisible by 99. Hence NN is divisible by 99. The difference between the odd and even sums is 00, which is divisible by 1111, so NN is divisible by 1111.

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