1. The problem is to calculate the sum of $-0.25$ and $-\frac{5}{6}$. 2. First, convert $-0.25$ to a fraction: $-0.25 = -\frac{1}{4}$. 3. Now, add the two fractions: $-\frac{1}{4} + (-\frac{5}{6})$. 4. Find a common denominator for $4$ and $6$, which is $12$. 5. Convert each fraction to have denominator $12$: $-\frac{1}{4} = -\frac{3}{12}$ and $-\frac{5}{6} = -\frac{10}{12}$. 6. Add the fractions: $-\frac{3}{12} + (-\frac{10}{12}) = -\frac{13}{12}$. 7. The result is an improper fraction, which can be written as $-1 \frac{1}{12}$ or approximately $-1.0833$. 8. Therefore, $-0.25 + (-\frac{5}{6}) = -\frac{13}{12}$ or $-1 \frac{1}{12}$.