A driver of a auto rickshaw sees a lorry 60 meters ahead of him After 30 seconds the lorry is 90 met...........

Quant (SSC) Time & distance

A driver of a auto rickshaw sees a lorry 60 meters ahead of him. After 30 seconds the lorry is 90 meters behind. If the speed of the auto rickshaw is 38 kmph, then what is the speed of the lorry?

23 kmph

25 kmph

20 kmph

18 kmph

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Explanation:

Relative speed = Total distance /Total time
= 60 + 90/30 = 5 m/s
∴ 5 m/s = 5 x 18/5 = 18 kmph
Now relative speed = speed of auto rickshaw – speed of lorry
or, 18 = 38 – speed of lorry
∴ Speed of lorry = 38 – 18 = 20 km/h