For each number from 1 to 99, Andrew wrote down the sum of its digits. What is the mean of the numbers Andrew wrote down?

Guest Aug 15, 2020

#1**+1 **

For each number from 1 to 99, Andrew wrote down the sum of its digits. What is the mean of the numbers Andrew wrote down?

**Hello Guest!**

\(\sum_{k=1}^n\ k=\frac{n^2+n}{2}\)

\(\sum_{k=1}^9\ k=\frac{9^2+9}{2}=45\)

The mean of the numbers from 1 to 99 is

\(\frac{10\cdot 45+9\cdot 45}{90+99}=\frac{855}{189}=\color{blue}4.\overline{523809}\)

!

asinus Aug 15, 2020