Divide the sum of

$\frac{15 m^{2} n^{3}}{5 m^{2} n^{2}}$

$=\frac{15 \times m \times m \times n \times n \times n}{5 \times m \times m \times n \times n}$

$=3 m^{(2-2)} n^{(3-2)}$

$=3 m^{0} n^{1}$

$=3 n$