In simplest surd form, the distance between A(1, 3) and B(5, 5) is:

$AB=\sqrt{(5-1{)}^2+(5-3{)}^2}=\sqrt{16+4}=\sqrt{20}$. Since $20=4\times 5,\sqrt{20}=2\sqrt{5}\approx 4.47$. Taking 16 out of the root instead of the perfect-square factor 4 gives $4\sqrt{5}$, which is far too large; 20 forgets the root altogether, and 6 is just 4 + 2.