numbers such as 9 and 25. factors of 9 are 1, 9, and 3 factors of 25 are 1, 25, and 5
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Prime numbers are numbers that have only 2 factors, 1 and it's self. Composite numbers are the opposite, they have 3 or more factors.
A number will always have 1 and itself as factors. And thus the only numbers with 2 factors will be prime numbers. The only prime numbers below 20 are 2, 3, 5, 7, 11, 13, 17 and 19, and thus these are the only numbers below 20 that have 2 factors.
42, 48 are the two numbers bigger than 40 that have 2 and 3 as their only prime factors.
The factors of 72 that are prime numbers are 2, 3, and 5. Prime numbers are numbers greater than 1 that can only be divided by 1 and themselves without leaving a remainder. In the case of 72, it can be broken down into the prime factors 2^3 * 3^2, where 2 and 3 are the prime factors.
A number has exactly three factors if and only if it is the square of a prime number.