To evaluate \( \frac{x}{y} \) where \( x = \frac{4}{5} \) and \( y = -0.1 \), we first convert \( y \) to a fraction:
\[ -0.1 = -\frac{1}{10} \]
Now we can substitute the values of \( x \) and \( y \) into the expression:
\[ \frac{x}{y} = \frac{\frac{4}{5}}{-\frac{1}{10}} \]
To divide by a fraction, we multiply by its reciprocal:
\[ \frac{4}{5} \div -\frac{1}{10} = \frac{4}{5} \times -10 \]
Now, calculate the multiplication:
\[ = \frac{4 \times -10}{5} = \frac{-40}{5} \]
Now, simplify \( \frac{-40}{5} \):
\[ = -8 \]
Thus, the value of \( \frac{x}{y} \) is:
\[ \boxed{-8} \]