Chapter 4 - Inverse, Exponential, and Logarithmic Functions - 4.6 Applications and Models of Exponential Growth and Decay - 4.6 Exercises - Page 479: 9

$k=\frac{1}{3}ln(\frac{1}{3})$

Step 1. State the model equation: $y(t)=y_0e^{kt}$ Step 2. For $y_0=60\ g$ and $y(3)=20$, we have $20=60e^{3k}$ Step 3. Solve the above: $e^{3k}=\frac{1}{3}\longrightarrow 3k=ln(\frac{1}{3}) \longrightarrow k=\frac{1}{3}ln(\frac{1}{3})$