Solution: Approximate $3π \approx 9.42$ and $5π \approx 15.71$. Whole numbers between them are 10, 11, 12, 13, 14, 15. Counting these gives $\boxed{6}$ numbers.A environmental scientist measures the concentration of PM2.5 in a city. If the concentration decreases by 5% every year due to emission controls, starting from 80 µg/m³, what will be the concentration after 5 years?

["Environmental Scientist’s Insight: Tracking PM2.5 Reduction Over Time", "Air pollution, particularly fine particulate matter (PM2.5), remains a critical concern for urban environments worldwide. A key metric in assessing public health risks is PM2.5 concentration, routinely monitored at levels such as 80 µg/m³. Environmental scientists analyze trends over time to evaluate the impact of pollution control measures.", "Consider a city where strict emission controls have reduced PM2.5 levels, starting at 80 µg/m³. Each year, the concentration drops by 5% due to improved policies and cleaner technologies. To project long-term air quality, understanding how such reductions unfold is essential.", "Using a simple exponential decay model, the concentration $ C(t) $ after $ t $ years is given by:\n$$\nC(t) = C_0 \ imes (1 - r)^t\n$$\nwhere $ C_0 = 80 $ µg/m³, $ r = 0.05 $ (5% annual reduction), and $ t = 5 $ years.", "Plugging in the values:\n$$\nC(5) = 80 \ imes (0.95)^5\n$$\nCalculating $ (0.95)^5 \approx 0.77378 $, so:\n$$\nC(5) \approx 80 \ imes 0.77378 = 61.9024 , \mu g/m³\n$$", "Thus, after five years of consistent emission controls, the PM2.5 concentration is projected to stabilize near 61.9 µg/m³—a significant decline from the initial 80 µg/m³—indicating measurable progress in improving urban air quality.", "This downward trend underscores the effectiveness of environmental policies. Like approximating $ 3\pi \approx 9.42 $ and $ 5\pi \approx 15.71 $ to guide urban planning estimates, precise modeling of PM2.5 decay supports data-driven decisions for healthier cities.", "Counting the whole numbers between 80 and $ 5\pi \approx 15.71 $—wait—this requires clarification. Actually, $ 5\pi \approx 15.71 $, so values between 80 and 15.71 lie between 16 and below—thus, no whole numbers fall between 80 and 15.71. But the original observation notes whole numbers 10 to 15 lying between $ 3\pi \approx 9.42 $ and $ 5\pi \approx 15.71 $, of which there are exactly 6: 10, 11, 12, 13, 14, 15.", "While not directly measuring PM2.5, this approximation shows how symbolic numbers help frame environmental benchmarks—just as precise concentration values guide real-world action.", "Final environmental projection:\nAfter 5 years, PM2.5 concentration drops to approximately 61.9 µg/m³, with 6 whole numbers lying between $ 3\pi $ and $ 5\pi $, symbolizing key milestones in air quality improvement.", "$$\n\boxed{6}\n$$"]