The expression \(\frac{x}{10} + \frac{3x}{8}\) simplifies to \(\frac{19x}{40}\). Since there are no restrictions on \(x\) that would make the denominator zero, the correct choice is:
\[
\boxed{\text{B. } \frac{x}{10} + \frac{3x}{8} = \frac{19x}{40} \text{, no numbers must be excluded.}}
\]