The second unit of the fifth grade math book focuses on the related contents of prime numbers and composite numbers, factors and multiples, which can directly examine students' mastery of these concepts, such as the judgment of prime numbers and composite numbers.

For example, a multiple of 9 within 50 is a multiple of 100 to 19, a factor of 25 and a factor of 65 start from 199, and five odd numbers are written continuously, and five even numbers are written continuously from 38. Non-zero natural numbers within dozens are even numbers, not composite numbers. What is odd, but not prime? A four-digit thousand.