What Is a Variable Expression

Variable : A symbol for a number whose value is not known.
To find the value of a variable expression, substitute the value of the unknown variable in the mathematical statement and simplify.
To frame a variable expression from a statement, following key points are used :
"a number" means a variable (like x) because you don't know which number they mean.
Sum, more than, increased by, plus, and together all mean addition (+)
Difference, less than, decreased by, and minus all mean subtraction (-)
Product, times, and of mean multiplication (x)
Quotient and divided by mean division (÷)