To find the value of the function \( y = \frac{1}{8}x - 13 \) when \( x = 4 \), we will substitute \( x \) with 4.
\[ y = \frac{1}{8}(4) - 13 \]
First, calculate \( \frac{1}{8}(4) \):
\[ \frac{1}{8}(4) = \frac{4}{8} = \frac{1}{2} \]
Now substitute that back into the equation:
\[ y = \frac{1}{2} - 13 \]
Convert \( -13 \) to a fraction for easier calculation:
\[ -13 = -\frac{26}{2} \]
Now we can add the fractions:
\[ y = \frac{1}{2} - \frac{26}{2} = \frac{1 - 26}{2} = \frac{-25}{2} \]
Now, we can convert \( \frac{-25}{2} \) to a decimal:
\[ \frac{-25}{2} = -12.5 \]
Thus, the value of the function when \( x = 4 \) is:
\[ y = -12.5 \]
The correct answer is:
- -12.5