To find out how much of the substance will be left after 18 days using the function \( A(t) = 135(0.917)^t \), we can substitute \( t = 18 \) into the equation.
\[ A(18) = 135(0.917)^{18} \]
Calculating \( (0.917)^{18} \):
\[ (0.917)^{18} \approx 0.302 \]
Now substitute this value back into the equation:
\[ A(18) \approx 135 \times 0.302 \approx 40.77 \]
Now round it to the nearest whole number:
\[ A(18) \approx 41 \]
However, none of the provided options of 29, 25, 28, or 30 match this result, so please double-check the function or calculations provided. If the original function or parameters change, we can re-evaluate accordingly!