1. **State the problem:** We need to find the sum of 327.79 and 321.58 and then estimate to check if the answer is reasonable. 2. **Add the numbers:** $$327.79 + 321.58 = 649.37$$ 3. **Estimate the sum:** Round each number to the nearest whole number: $$328 + 322 = 650$$ 4. **Compare the exact sum and estimate:** The exact sum is 649.37 and the estimate is 650. 5. **Conclusion:** Since 649.37 is very close to 650, the answer is reasonable. **Final answer:** $$\boxed{649.37}$$ **Estimated sum:** 650 Thus, the answer is reasonable.