To express the number 21 as a fraction in its simplest form, we consider it as 21 over 1. This is because any whole number can be represented as a fraction by placing it over 1. Therefore, we can write:
21 = 21/1
This fraction is already in its simplest form because the numerator (21) and the denominator (1) have no common factors other than 1. Thus, the simplest form of 21 as a fraction is 21/1.