2. There are seven small balls of the same size, numbered $1,2, \cdots, 7$. Three balls are randomly selected from them. The probability that the sum of the numbers on the three selected balls is odd is $\qquad$.
2. $\frac{16}{35}$.

Notice,
the sum of the numbers on the three balls is odd
$\Leftrightarrow$ the three numbers are either all odd or one odd and two even.
Thus, the required probability is $\frac{\mathrm{C}_{4}^{3}+\mathrm{C}_{4}^{1} \mathrm{C}_{3}^{2}}{\mathrm{C}_{7}^{3}}=\frac{16}{35}$.