Given: Two numbers- 15 and 21.
Required: To determine the GCF.
Explanation: GCF is the greatest common factor between two or more numbers. We can determine the GCF by prime factorization of 15 and 21 as follows-
[tex]\begin{gathered} 15=3\times5 \\ 21=3\times7 \end{gathered}[/tex]The greatest common factor among the prime factorization of 15 and 21 is 3.
Hence, the GCF is 3.
Final Answer: The GCF of 15 and 21 is 3.
