A fraction is said to be in simplest form if its numerator and denominator are relatively prime, that is, they have no common factors other than . (Some books use "written in lowest terms" to mean the same thing.)
So, is in simplest form, since and have no common factors other than . But is not; and have a common factor .
To write in simplest form, divide both the numerator and denominator by the greatest common factor, in this case :
So in simplest form is .
This is known as reducing fractions.