Is a problem solving method that uses that fact that any number or expression can be multiplied by one without changing it's value.
So 81/9=9 so area of pantry is 9
3 times x=9
3x=9
divide both sides by 3
x=3
legnth/width=3
Whether it can be equal to that value. For example, if 5 is greater than x, then an open circle will be useful. If 5 is greater than or equal to x, a closed circle will be used.