Question

28. A car travels half of the length of straight line
motion with a speed of 60 km/h. The remaining
part of the distance is covered with speed
40 km/h for half of the time of remaining journey
and with speed 20 km/h for the other half of the
time. The average speed of the car for the entire
journey is
(1) 30 km/h
(2) 40 km/h
(3) 48 km/h
(4) 45 km/h​

Answer 1

Answer:

the average speed will be 30km/h

Answer 2

Answer:

Average speed will be 37.03 m/sec

Which is not given in all option

Explanation:

Let the total distance is d

It is given that car travel half of the distance with the speed of 60 km/hr

So time taken to cover half of the distance $t=\frac{distance}{time}=\frac{\frac{d}{2}}{60}=\frac{d}{120}hr$

Remaining distance $d-\frac{d}{2}=\frac{d}{2}$

Now half of the remaining distance is traveled by speed of 40 km/hr

So time taken to cover this distance $t=\frac{distance}{time}=\frac{\frac{d}{4}}{40}=\frac{d}{160}hr$

After that remaining distance is traveled by 20 km/hr

So time $t=\frac{distance}{time}=\frac{\frac{d}{4}}{20}=\frac{d}{80}hr$

So total time

$=\frac{d}{120}+\frac{d}{160}+\frac{d}{80}=0.0270$

We know that average speed $=\frac{total\ distance}{total\ time}=\frac{d}{0.0270d}=37.03m/sec$

So average speed will be 37.03 m/sec

Which is not given in all option