1. **State the problem:** The rabbit population this year is 200% of last year's population. This year there are 900 rabbits. We need to find how many rabbits there were last year. 2. **Understand the percentage:** 200% means twice the amount of last year. So if last year's population is $w$, then this year's population is $2 \times w$. 3. **Set up the equation:** $$900 = 2 \times w$$ 4. **Solve for $w$:** Divide both sides by 2: $$\frac{900}{\cancel{2}} = \frac{2 \times w}{\cancel{2}}$$ $$450 = w$$ 5. **Answer:** There were 450 rabbits last year. This means the population doubled from 450 to 900 this year.