Question 117738
First, list the prime factorization of each number. If you need help with prime factorization, check out this <a href="http://www.algebra.com/algebra/homework/divisibility/factor-any-number.solver">solver</a>.



105: 3*5*7



147: 3*7*7



Now highlight the factors that the two numbers share in common



105: <font color="red">3</font>*5*<font color="red">7</font>



147: <font color="red">3*7</font>*7



Now pick one set of these highlighted numbers and multiply


3*7=21


So the GCF of 105 and  147  is 21



So the answer is A)