Number theoryDifficulty 5.0AIMEProve itSoviet Union
Problem: All two digit numbers from 19 to 80 inclusive are written down one after the other as a single number N=192021…7980. Is N divisible by 1980?
Solution
Solution: 1980=22325⋅11. N is obviously divisible by 22 and 5. The digits in odd position are 9+(0+1+2+…+9)+(0+1+2+…+9)+…+(0+1+2+…+9)+0=9+6⋅45=279. The digits in even position are 1+(2+2+…+2)+(3+3+…+3)+…+(7+…+7)+8=9+10(2+3+…+7)=279. So the sum of the digits of N is 2⋅279 which is divisible by 9. Hence N is divisible by 9. The difference between the odd and even sums is 0, which is divisible by 11, so N is divisible by 11.
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Source: MathNet,
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