1. **Problem statement:** We need to understand what it means for two numbers to be equal to $n$ decimal places and then check if $3.6218$ is approximately equal to $3.6221$ to 3 and 4 decimal places. 2. **Definition:** Two numbers $a$ and $b$ are equal to $n$ decimal places if their difference is less than $0.5 \times 10^{-n}$. This means: $$|a - b| < 0.5 \times 10^{-n}$$ 3. **Calculate the difference:** $$|3.6218 - 3.6221| = 0.0003$$ 4. **Check equality to 3 decimal places:** The tolerance is: $$0.5 \times 10^{-3} = 0.0005$$ Since $0.0003 < 0.0005$, we conclude: $$3.6218 \approx_3 3.6221$$ 5. **Check equality to 4 decimal places:** The tolerance is: $$0.5 \times 10^{-4} = 0.00005$$ Since $0.0003 \not< 0.00005$, we conclude: $$3.6218 \not\approx_4 3.6221$$ **Final answer:** $3.6218$ is equal to $3.6221$ to 3 decimal places but not to 4 decimal places.