Question

18 plus what number equals 20?
18 + 15 = ?

Explanation:

Step1: Define unknown variable

Let the unknown number be $x$. The equation is $18 + x = 20$.

Step2: Solve for unknown variable

Subtract 18 from both sides:
$x = 20 - 18$
$x = 2$

Step3: Correct the addition problem

Replace 15 with $x=2$, calculate the sum:
$18 + 2 = 20$

Answer:

The unknown number is 2, and $18 + 2 = 20$