## Problem

Dear friends and visitors of the site!
Here is our November Problem:

How many 13-digit numbers have an odd sum of digits?

## Answer Submission Is Not Available

Form is valid through November 2018

## Solution

Dear friends and visitors of the site.

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 10^{12}/2

Thank you again and tr<y our December Problem!

Yours,

Serge Hazanov

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