How Average Of You

Relevant wiki: Mean (Average)

We have $(x_1+ x_2 + x_3 +\cdots+x_{10}) / 10 =10$ .

Lets say $x_1+ x_2+\cdots x_{10} = S$ .

So $S = 100$ , and let $S'$ denote the sum of the nine remaining numbers.

So $S' / 9 = 9$ , or $S' = 81$ .

And $S=100$ . So $S-S' = 19$ . So the excluded number is $\boxed{19}$ .