Question

A train travels the first 15km at uniform speed of 30km/h next 10 km at uniform speed of 20 km/h and final 75 km at uniform speed of 50 km/h . the average speed for the entire journey of the train ?

Answer 1

average speed=total distance travelled/total time taken.
Vav=s1+s2+s3/(s1/v1)+(s2/v2)+(s3/v3)
Vav=15+10+75/(15/30)+(10/20)+75/50
=100/0.5+0.5+1.5==》100/2.5
==》40km/hr

Answer 2

for the first 15km distance,time taken:-
t1 = 15km/30km/h
= 0.5h
for the next 10km,time taken:-
t2 = 10km/20km/h
= 0.5h
for the last 75km,time taken by the train is:-
t3 = 75km/50km/h
= 1.5h
average speed= total distance traveled/total time taken
=(30+20+50)km/(0.5+0.5+1.5)s
=100km/2.5h
=40km/h