To find the amount of Radium-226 remaining after seven years, we can use the given decay formula:
\[ A(t) = 450e^{-0.0004t} \]
We need to evaluate \( A(7) \):
\[ A(7) = 450e^{-0.0004 \times 7} \]
First, calculate \( -0.0004 \times 7 \):
\[ -0.0004 \times 7 = -0.0028 \]
Now substitute this value into the equation:
\[ A(7) = 450e^{-0.0028} \]
Next, we calculate \( e^{-0.0028} \):
Using a calculator or mathematical software:
\[ e^{-0.0028} \approx 0.9972 \]
Now, substitute back to find \( A(7) \):
\[ A(7) = 450 \times 0.9972 \approx 448.74 \]
Finally, rounding this to the nearest tenth gives:
\[ \boxed{448.7} \text{ grams} \]
Thus, after seven years, approximately 448.7 grams of Radium-226 will remain.