3. All other multiples of 3 are not prime.
Prime and odd numbers
To find prime numbers less than 100, the sieve of eratosthenes filters out 1 and all multiples of 2, 3, 5, and 7. All remaining numbers less than 100 are primes.
Everything up to and including 20.
42 and its next three multiples.
There are 35 prime numbers less than 150.
Numbers that are prime can't be multiples of 10. Multiples of 10 can't be odd.
There are 48 prime numbers less than 220.
There are 27 prime numbers less than 102.
There are 15 prime numbers less than 50
They are 239 prime numbers less than 1500.
There are 25 prime numbers less than 100