1. The problem states: The sum of a number and 18 is equal to 42. 2. Let the number be $x$. The phrase "sum of a number and 18" translates to $x + 18$. 3. According to the problem, this sum equals 42, so the equation is: $$x + 18 = 42$$ 4. To solve for $x$, subtract 18 from both sides: $$x + 18 - 18 = 42 - 18$$ $$x = 24$$ 5. Therefore, the number is 24. This is a simple linear equation where you isolate the variable by performing inverse operations.