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?
+15001 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
\(\frac{10\cdot 45+9\cdot 45}{90+99}=\frac{855}{189}=\color{blue}4.\overline{523809}\)
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