Simplify $(2 x-5 y)^{3}-(2 x+5 y)^{3}$

We have,

$(2 x-5 y)^{3}-(2 x+5 y)^{3}$

$=\{(2 x-5 y)-(2 x+5 y)\}\left\{(2 x-5 y)^{2}+(2 x-5 y)(2 x+5 y)+(2 x+5 y)^{2}\right\}$

$\left[\because a^{3}-b^{3}=(a-b)\left(a^{2}+a b+b^{2}\right)\right]$

$=(2 x-5 y-2 x-5 y)\left(4 x^{2}+25 y^{2}-20 x y+4 x^{2}-25 y^{2}+4 x^{2}+25 y^{2}+20 x y\right)$

$=(-10 y)\left(2 x^{2}+25 y^{2}\right)$

$=-120 x^{2} y-250 y^{3}$