Factorise:

(i) H.C.F. of $16 a^{2}$ and $24 a b$ is $8 a$.

$\therefore 16 a^{2}-24 a b=8 a(2 a-3 b)$

(ii) H.C.F. of $15 a b^{2}$ and $20 a^{2} b$ is $5 a b$.

$\therefore 15 a b^{2}-20 a^{2} b=5 a b(3 b-4 a)$

(iii) H.C.F. of $12 x^{2} y^{3}$ and $21 x^{3} y^{2}$ is $3 x^{2} y^{2}$.

$\therefore 12 x^{2} y^{3}-21 x^{3} y^{2}=3 x^{2} y^{2}(4 y-7 x)$