\[\left( x^{25}\ :x^{5} \right) \cdot x^{10}\ :x^{3} =\]
\[= x^{25 - 5} \cdot x^{10}\ :x^{3} =\]
\[= x^{20} \cdot x^{10}\ :x^{3} =\]
\[= x^{20 + 10 - 3} = x^{27}.\]