A train travels for 8 hrs. The first half of the total distance at 30 km/hr and the second half at 50 km/hr. The total distance travelled is

Time taken to cover first half distance ${\text{t}}_1=\frac{\frac{d}{2}}{30}=\frac{d}{60}$

Time taken to cover second half distance ${\text{t}}_2=\frac{\frac{d}{2}}{50}=\frac{d}{100}$

${\text{t}}_1+{\text{t}}_2=8\text{hr}$

$\frac{d}{60}+\frac{d}{100}=8\Rightarrow \frac{160d}{6000}=8$

$\therefore d=\frac{4800}{16}=300\text{km}$