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



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 1012 /2

Thank you again and tr<y our December Problem!


Serge Hazanov