To find the greatest common factor we need to decompose the number into prime numbers
To find the GCF we see which numbers are being repited in both numbers
[tex]\begin{gathered} 45=5\cdot3\cdot3 \\ 20=5\cdot2\cdot2 \end{gathered}[/tex]
the greatest common factor between 45 and 20 will be 5, because its the only factor after decomposing the number that repeats on both.
