a letter
it could be a b c d e f g h i j k l m n o p q r s t u v w x y z anything
a variable expression in math is a symbol that is in a sentence
The question should be what is the variable in the following expression? (n + 5) The answer: (n) represents the variable. See: Variable(English definition) then look up the Math/Algebra definition.
In this case N is variable for a math sentence. If the equation is n+7=9 n would equal 2.
Take the value of each variable in the expression and replace the variable by its value. Then do the math!
The unknown variable in the expression: 6n+3 is n
n/3
n is the variable.
It is n
N
In the expression 6n+3 the variable or the unknown is n
n+81
N + 5 With "N" being the variable.