$$-\frac{8\dot{x}}{100}+\frac{20000-x}{75}=800; \frac{6x}{75}+\frac{20000-x}{75}=800; \frac{6x-x+20000}{75}=800; 5x+20000=800\times75; 5x=60000-20000; 5x=40000; x=8000544$$
