The expression is “twelve is one fifth of a number”.

Let n represent the number.

The word “is” can be represented as “ = ” symbol.

Rewrite the sentence into expression as \ \

The equation is

According to multiplication property of equality:

For any number a, b and c, If a = b then ac = bc.

                     (Multiply each side by 5)

                               (Cancel common terms)

n = 60                                     (Divide: )

The number is 60.