The exponential decay function for the mass of the substance (y) in terms of the number of years (t) since 2000 is
![\[ y = 400 \cdot (1 - 0.039)^t \]](https://img.qammunity.org/2024/formulas/mathematics/high-school/fes5i06ww1hr0a5cs8wfzalhlli1djkgnv.png)
The exponential decay model can be expressed using the formula:
![\[ y = y_0 \cdot (1 - r)^t \]](https://img.qammunity.org/2024/formulas/mathematics/high-school/20orgshbmvm4f2xksjav5f20c5kpklwu51.png)
where:
- y is the final amount,
-
is the initial amount,
- r is the decay rate per time period,
- t is the number of time periods.
In this scenario:
-
milligrams (initial mass in 2000),
- r = 0.039 (3.9% decay per year),
- t is the number of years since 2000.
Therefore, the exponential decay function for the mass of the substance (y) in terms of the number of years (t) since 2000 is:
![\[ y = 400 \cdot (1 - 0.039)^t \]](https://img.qammunity.org/2024/formulas/mathematics/high-school/fes5i06ww1hr0a5cs8wfzalhlli1djkgnv.png)