Twice A Number

The
number "x" is represented as a letter because it is a variable.
This is the idea of the type of math that is named algebra. Replacing the
variables with single letters (sometimes English alphabet, sometimes Greek alphabet - because
the Greeks had a big role in inventing algebra, so we still use their alphabet today) is a
shorthand. When we work with long or complicated expressions involving one or more variables,
this shorthand makes it easier to follow through the steps on paper at a glance, concentrating
it down to the real essentials. It also saves on ink and time!

An example of
a variable that "x" might represent is the number of oranges a boy called Sam eats in
a week. If his friend Jenny always eats twice as many oranges in a week as Sam, then we can say
that she eats "2x" oranges in a week. It is a variable because it
varies each week, but the relationship between what the two friends eat always stays the same.
You can see that "x" is a shorthand way of saying "number of oranges eaten per
week". It is like when you write $ to mean United States Dollars. It makes comparing prices
at a glance easier.