1. **State the problem:** We are given the population size formula $$P = 1200 \cdot (0.85)^{12t}$$ where $t$ is in years. We need to find the monthly percent decay rate of the population $P$. 2. **Understand the formula:** The formula shows the population decays by a factor of $0.85^{12t}$ over $t$ years. Since $t$ is in years, $12t$ represents the number of months. 3. **Rewrite the formula in terms of months:** Let $m = 12t$ be the number of months. Then, $$P = 1200 \cdot (0.85)^m$$ 4. **Identify the monthly decay factor:** The population decays by a factor of $0.85$ each month. 5. **Calculate the monthly percent decay rate:** The decay rate per month is $$\text{Decay rate} = (1 - 0.85) \times 100 = 0.15 \times 100 = 15\%$$ 6. **Final answer:** The monthly percent decay rate in $P$ is **15%**.