Answer: There will be 0.04 grams will be left in the sample after 20 years.
Step-by-step explanation:
Since we have given that
[tex]A(t)=400e^{-0.46t}[/tex]
Here, Initial amount of radioactive substance = 400 grams
We need to find the amount of substance that will be left in the sample after 20 years.
So, t = 20 years
So, it becomes,
[tex]A(t)=400e^{-0.46\times 20}\\\\A(t)=400e^{-9.2}\\\\A(t)=0.04\ grams[/tex]
Hence, there will be 0.04 grams will be left in the sample after 20 years.