Question

A car travels from A to B at a speed of 20 km/hr
and returns at a speed of 30 km/hr . The average
speed of the car for the whole journey is
(a) 25 km/hr
(c) 50 km/hr
(b) 24 km/hr
(d) 5 km/hr
Im with​

Answer 1

Answer:

$\frac{20 + 30}{2} = \frac{50}{2} = 25 \: km \: per \: hr$

Answer 2

Let the distance from A to B be = (x) km

We know, t = s/v

If total time taken = (T) hrs,

∴ t1 = x/v1 = x/20 h (A to B)

t2 = x/v2 = x/30 h (B to A)

Now, average speed = (Total distance)/(Total time)

⇒ v (bar) = (x + x)/T

⇒ v (bar) = (2x)/(t1 + t2)

⇒ v (bar) = (2x)/(x/20 + x/30)

⇒ v (bar) = 2/(50/600)

⇒ v (bar) = 24

So, average speed of the car was 24 kmph.

NB:

Average speed is not always equal to ½(v + u).