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
![\begin{gathered} 45=5\cdot3\cdot3 \\ 20=5\cdot2\cdot2 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/mir7xad128jx7jlwpoh2iv8e1q8zpg84mx.png)
the greatest common factor between 45 and 20 will be 5, because its the only factor after decomposing the number that repeats on both.