Answer :
If you want all of the common factors then there are 15, 5, 3 and 1 however the highest common factor would therefore be 15.
[tex]15|5\\3 \ |3\\1 \\\\15=3*5 \\\\ 75|5\\15|5\\3 \ |3 \\1 \\\\75=3*5^2 \\\\ H.C.F(15;75)=3*5=15[/tex]