#### Answer

6 is the answer.
6 is given in both sets, 12 and 18
This means that 6 is a common factor of 12 and 18 as both these numbers can be divisible by 6 to produce an integer( a whole number)

#### Work Step by Step

When numbers are divided by their integers it will produce a new integer. An example of this is 20\2=10
The factors of 12 are {1, 2, 3, 4, 6, 12}, and the factors of 18 are {1, 2, 3, 6, 9, 18}. The greatest common number in those sets is 6 as it is the biggest factor that both 12 and 18 share.
1,2,3 and 6 are factors that both 12 and 18 share but 6 is the biggest number in that set of numbers.