Question

A car travels from one town to the other with average speed 20 km/hr. If the first half is travelled at average speed
30 km/hr., then the average speed of the car in the other half will be:
(A) 30 km/hr
(B) 40 km/hr
(C) 50 km/hr
(D) 15 km/hr
​

Answer 1

Explanation:

Let the distance between one town and another town be x. Average speed is 20 km/hr. First half has average speed of 30 km/hr.

Time taken for first half is distance/speed => x/30 whereas let's take speed for return trip be u so time taken for second half is distance/speed => x/u. Hence average speed =Total distance/total time taken =>

20 = 2x/(x/30+x/u) => 20 = 2x/(30+u)x/30u => 20 = 60u/(30+u) => 600+20u=60u=>600 =

60u-20u=40u =>u =600/40 = 15 km/hr. So option(D) is correct. Hope it helps you.

Answer 2

Answer:

$option \: (d) \: is \: correct \:$

$15 \frac{km}{hr}$