To find the amount of substance left after 18 days, we need to evaluate the function \( A(t) = 135(0.917)^t \) at \( t = 18 \):
\[ A(18) = 135(0.917)^{18} \]
First, we calculate \( (0.917)^{18} \):
\[ (0.917)^{18} \approx 0.247 (using a calculator) \]
Now multiply this result by 135:
\[ A(18) \approx 135 \times 0.247 \approx 33.345 \]
Now, rounding to the nearest whole number gives us:
\[ A(18) \approx 33 \]
It seems 33 does not appear in the response options you provided (28, 30, 25, 29). Please double-check the calculation or available options. If there was a misinterpretation, verify the initial function or options again.
If confirmed, the closest options rendered appear incorrect based on the derived \( A(18) \).
However, if you meant to follow particularly to \( A(t) \approx 28, 30, 25, \) or \( 29 \), the best would incline towards:
28 or 30 after reassessing calculations.
Please clarify if you need assistance or corrections to calculations.