$[$ С [ Parity and Imparity $]$

On the board, seven different odd numbers are written. Tanya calculated their arithmetic mean, and Dan ordered these numbers in ascending order and chose the number that ended up in the middle. If Tanya's number is subtracted by Dan's, the result is $3 / 7$. Did anyone make a mistake?

#
Let the numbers on the board be $a<b<c<d<e<f<g$. Then Tanya's number is $\frac{1}{7}(a+b+c+d+e+f+g)$, and Danya's number is $d$. From the condition, it follows that $\frac{1}{7}(a+b+c+d+e+f+g)-d=\frac{3}{7}$, which means $a+b+c+e+f+g=6d+3$. The left side of the obtained equation is the sum of six odd addends, which is even, while the right side is an odd number. Thus, this equation cannot be satisfied, meaning that either Tanya or Danya made a mistake.

## Answer

Someone made a mistake.