For a number  to be relatively prime with , . Equivalently,  and  cannot share a prime factor. , so any number that does not have either  or  as a factor is a correct choice.

We can eliminate  and , since  is a factor of each; we can eliminate and , since  is a factor of each. But , so .  is the correct choice.

To find the greatest common factor (GCF), you need to determine the factor that both numbers share that is of the greatest value. List the factors of each number and identify the largest number in value that is in both lists:

The GCF of 20 and 36 is 4 since it is the largest number in value that shows up in both lists.

To find the greatest common factor (GCF), you need to determine the factor that all three numbers share that is of the greatest value. List the factors of each number and identify the largest number in value that is in all three lists:

The GCF of 15, 30, and 40 is 5 since it is the largest number in value that shows up in all three lists.

Factor each number:

Identify the common factors (). The greatest common factor is the product of all of the common factors.

The greatest common factor of two numbers is the largest shared factor that the two numbers have in common. 

The factors of 36 are 

The factors of 108 are 

The largest shared factor between 36 and 108 is 36. Therefore, 36 is the correct answer. 

Find the greatest common factor (GCF) of 56 and 64.

To find the GCF, first find the prime factors of our two numbers:

Now, to find our GCF, we need to find the prime factors our two numbers have in common.

In this case, we have three 2's in common. In other words, we have an 8 in common. So, our GCF is 8.

Find the greatest common factor (GCF) of 324 and 56.

The GCF is the largest number which will can be evenly divided from either number.

To find the GCF, first find the prime factors of both numbers. A good trick here is to start by pulling a two out of each number until you cannot pull out a two anymore.

Beginning with 56

So the prime factorization of 56 looks like:

Next, let's do 324.

So for 324 we get:

Now, the GCF can be found by taking the prime factors which are common to both numbers. Now, in this case, we have two 2's in each number, and nothing else. This means that the GCF of 56 and 324 is 

So our answer is 4

Fully factor  and . Circle the factors in common:

Multiple the common factors to find the greatest common factor:

25 has three factors: 1, 5, and 25. Their product is 

None of the choices are prime, so each has at least three factors. The question, then, is which one has only three factors?

We can eliminate four choices by showing that each has at least four factors - that is, at least two different factors other than 1 and itself:

Each, therefore, has at least four factors.

However, the only way to factor 121 other than  is . Therefore, 121 has only 1, 11, and 121 as factors, and it is the correct choice.