variables and expression: the value in cents of 5 quarters

A variable is an unknown or an assumed value of any thing
till it is known or it is solved..

A variable is any value
varying.

A fixed value is a constant or a known
value.

An expression is a mathematical operation or
operations ( like addition, subtraction, multiplication, or division , powers, roots,
factorials or any  defined compositions)  etc on numbers or
variables.

Here between the number   cents and 5 quarters
we can establish a relation:

Let  5 quarters be equal to x
cents.

So x = 5quarters..........(1) is the relation or
equation.

x is the variable. 5quarters is the known
value.

x can be any thing for which the relation is true or
not true.

We know 1 quarter  = 25 cents. So
,

x cents = 5*25cents , here 5*25 cents is an expression.
So,

x cents = 5*25cents.

x
cents = 125 cents.

x = 125. So x = 125 is the only value
for which reation or equation at (1) is true or holds good. (For any other value of x
the relation does not hold good. Or the relation is not true for values other than
125.)