Core Formula/Logic: Population growth rate = [(Ending population ÷ Starting population)^(1/number of years) - 1] × 100, where 1990-99 covers 9 years.
Step-by-Step Solution: 1. India's population in 1990: 846.4 million (Census 1991 provisional).
2. India's population in 1999: 1,000 million (1 billion estimate).
3. Growth factor = 1,000 ÷ 846.4 = 1.1815.
4. Annual growth factor = 1.1815^(1/9) = 1.0187.
5. Annual growth rate = (1.0187 - 1) × 100 = 1.87% ≈ 1.8%.
Common Pitfall: Using 10 years instead of 9 (1990-99 inclusive) gives 1.81%, still closest to 1.8%. Misreading 1990-99 as 1990-2000 yields 2.0%.
Shortcut/Takeaway: For decade growth approximations, 1.8% annual growth doubles population in 39 years (Rule of 70: 70 ÷ 1.8 ≈ 39). India's growth rate declined from 2.2% (1980s) to 1.8% (1990s).