Let $a_0, a_1, \ldots, a_{23}$ be real numbers such that
$
\left(1+\frac{2}{5} x\right)^{23}=\sum_{i=0}^{23} a_i x^i
$
for every real number $x$. Let $a_r$ be the largest among the numbers $a_j$ for $0 \leq j \leq 23$.
Then the value of $r$ is $\_\_\_\_$
