Question

A car travels from a point A to B point at constant speed of 50km/h .on return journey along the same paths, it travels at a constant speed 40km find it's distance travelled displacement and average speed

Answer 1

average speed=Total distance/Total time 
the total distance travelled by the car = D+D= 2D 
Speed = Distance/Time
S1 = D/t1 => t1 = D/S1 
S2 = D/t2 => t2 = D/S2 

average speed=Total distance/Total time 
= 2D/(t1+t2) = 2D/(D/S1+D/S2) = 2S1*S2/(S1+S2

S1 = 20km/hr   &S2 = 30km/hr

 av. speed = 2 x 20 x 30/20+30

                   = 24km/hr

Answer 2

in first case the car covers 30km the speed of 4okm/h
speed=distance
------------
time
or, 40=30 by t
or,40t=30
or,t=30 by 40
or, t=3 by 4

in second case the car covers 30 km the speed of 20km/h.
v=s by t2
or, 20=30 by t2
or, 20 t2= 30
or, tw=20 by 30 = 2 by 3

therefore, total speed = 3 by 4 +3 by 2
= 3+6 by 4. = 9 by 4


total distance = 30+30= 60
therefore, avg. speed =60 multiplied by 4. by 9
=80 by 3 =26.6