1. **Stating the problem:** Solve for the missing number in the equation $15 - \_ = 4$. 2. **Formula and rules:** To find the missing number, isolate it by performing inverse operations. Since subtraction is involved, add the missing number to both sides or rearrange the equation. 3. **Intermediate work:** $$15 - x = 4$$ Subtract 4 from both sides: $$15 - x - 4 = 4 - 4$$ $$15 - x - 4 = 0$$ Simplify: $$11 - x = 0$$ Add $x$ to both sides: $$11 = x$$ Alternatively, subtract 4 from 15: $$x = 15 - 4$$ $$x = 11$$ 4. **Explanation:** The missing number is the amount you subtract from 15 to get 4. Since $15 - 11 = 4$, the missing number is 11. **Final answer:** $11$