1. **State the problem:** Solve for $x$ in the equation $x + 40 = 90$. 2. **Formula and rules:** To isolate $x$, subtract 40 from both sides of the equation. 3. **Intermediate work:** $$x + 40 = 90$$ Subtract 40 from both sides: $$x + \cancel{40} - \cancel{40} = 90 - 40$$ 4. **Simplify:** $$x = 50$$ 5. **Explanation:** We moved 40 to the right side by subtracting it from both sides, which leaves $x$ alone on the left. The result is $x = 50$. **Note:** The value $x=77$ mentioned is not consistent with the equation $x + 40 = 90$.