Question

A car moves from A and B with a speed of 30km/h and from B and A with speed of 20km/h.what is the average speed of the car

Answer 1

let the distance from A to B be x km
time taken from A to B = x/30 hour
time taken from B to A= x/20 hour
so, avg speed = total distance/total time
                       =(x+x)/(x/30+x/20)
                       =2x * 12/x                                        x/30+x/20= (2x+3x)/60 = x/12
                       =24  km/h

Answer 2

A to B with a speed = 30 Km/h

B to A with a speed = 20 Km/h

Let distance from A to B and B to A = x Km

We know, speed = distance/time

or time = distance/speed and distance = speed × time

So, time from A to B = x/30 h

and B to A = x/20 h

And we know average speed = total distance/total time

(x + x)/(x/30 + x/20) = (2x)/(2x + 3x)/60

(2x)/(6x/60) = (2x)/(12x) = 24 Km/h is the average speed of the car.