How do you get the greatest common factor of 105/20?

Find the prime factor of 105
105 = 3 × 5 × 7

Then Find the prime factor of 20 20 = 2 × 2 × 5

And last Multiply all the common factors obtained in steps i) and ii) above to find the gcf:
GCF = 5

Add the cents that add the left overs to get the first number