Q.

Two cars are moving in the same direction with the same speed 30km/hr. They are separated by a distance of 5 km, the speed of a car moving in the opposite direction if it meets these two cars at an interval of 4 minutes, will be 

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a

45 km/hr

b

40 km/hr

c

30 km/hr

d

15 km/hr

answer is B.

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Detailed Solution

The two cars (say A and B) are moving with same velocity, the relative velocity of one (say B) with respect to the other

A,vBA=vBvA=vv=0

So the relative separation between them 5km always remains the same.

Now if the velocity of car (say C) moving in opposite direction to A and B, is vC relative to ground then the velocity of car C relative to A and B will be  vrel=vCv 

But as v is opposite to vC ,so

vrelvC30=vC+30km/hrSo, the time taken by it to cross the cars A and B

t=dvrel

460=5vC+30

vC=45 km/hr

 

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