13. \( \frac{3}{15}+\frac{-5}{6} \)

Let's simplify \( \frac{3}{15} \) first. This fraction can be reduced to \( \frac{1}{5} \) since both the numerator and the denominator can be divided by 3. Now, to add \( \frac{1}{5} \) and \( \frac{-5}{6} \), we need a common denominator. The least common multiple of 5 and 6 is 30. Now, convert both fractions: \( \frac{1}{5} = \frac{6}{30} \) \( \frac{-5}{6} = \frac{-25}{30} \) Now add them together: \( \frac{6}{30} + \frac{-25}{30} = \frac{6 - 25}{30} = \frac{-19}{30} \) So, the result is \( \frac{-19}{30} \).