The annual rate of decay is r = 0.87
We know that if a quantity has a half-life of T, then the rate of decay must be such that:
r^T = 0.5
This means that when a time T passes, the original quantity becomes half of what it originally was.
In this case we know that the half-life is T = 5 years, then we have:
r^5 = 0.5
r = 0.5^(1/5) = 0.87
Then the annual rate of decay is r = 0.87, and if the initial quantity is 18 grams, we can write the exponential decay as:
f(t) = 18g*(0.87)^t
If you want to learn more about exponential decays, you can read:
https://brainly.com/question/11464095
