How many 13-digit numbers have an odd sum of digits?
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Thank you for your effort and your answers.
The first to send a correct solution was Andrew. Let's congratulate him!
Here is my solution:
Let’s cut all these numbers in couples of consecutives.
In such a couple one numbers has an even sum of digits and another – an odd.
So half of them has an odd sum,so we get 9 x 1012 /2
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