A boy walks to his school at a distance of 6 kilometre with constant speed of 25 KM walk back with a constant speed of 75 kilometre per hour. His average speed for round trip expressed in kilometre per hour??

Dear student,
While going to school:
distance, s1=6 km
speed, v1=25 km/hr
time, t1= $\frac{s1}{v1}=\frac{6}{25}$

While coming back:
s2= 6 km
v2= 75 km/hr
t2= $\frac{s2}{v2}=\frac{6}{75}$

Now, 
Average speed, v= Total distance/ Total time

v=  $\frac{6+6}{{\displaystyle \frac{6}{25}}+{\displaystyle \frac{6}{75}}}$= $\frac{12}{{\displaystyle \frac{24}{75}}}$= $\frac{75}{2}$= 37.5 km/hr

Regards