The prime numbers less than 20 are:
\[ 2, 3, 5, 7, 11, 13, 17, 19 \]
If we denote the set of these prime numbers as \( p \), then \( p' \) typically refers to the complement of \( p \) within a certain universal set. If we consider the universal set to be the integers less than 20, then \( p' \) would contain all the integers less than 20 that are not prime.
The integers less than 20 are:
\[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 \]
From these, we exclude the primes to find \( p' \):
The numbers that are not prime less than 20 are:
\[ 0, 1, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18 \]
Thus, \( p' \) contains:
\[ 0, 1, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18 \]