Which of the following is not a factor of 18?

To determine which number is not a factor of 18, it's important to first understand what factors are. Factors of a number are integers that can divide that number without leaving a remainder.

When examining the number 18, we can identify its factors: they are the numbers that evenly divide into 18. These factors include 1, 2, 3, 6, 9, and 18.

Looking at the choices provided, we see that:

• 1 is a factor of 18 because 18 divided by 1 is 18.

• 2 is a factor of 18 because 18 divided by 2 is 9.

• 9 is a factor of 18 because 18 divided by 9 is 2.

On the other hand, 4 is not a factor of 18 because when 18 is divided by 4, it does not yield an integer; specifically, 18 divided by 4 results in 4.5, which indicates that it does not divide evenly.

This is why the answer identifies 4 as the number that does not qualify as a factor of 18.