1. **State the problem:** Solve for $y$ in the equation $125 + 548 - 321 + y = 999$. 2. **Combine like terms:** First, add and subtract the constants on the left side. $$125 + 548 = 673$$ $$673 - 321 = 352$$ So the equation becomes: $$352 + y = 999$$ 3. **Isolate $y$:** Subtract 352 from both sides to solve for $y$. $$y = 999 - 352$$ 4. **Calculate the result:** $$y = 647$$ **Final answer:** $y = 647$