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Two cars travel in the same direction at an equal speed of 30 km/h and are separated by 5 km. A third car moves in the opposite direction. If it meets the two cars 4 minutes apart, what is the speed of this third car?

1. 40 km/hr
2. 30 km/hr
3. 45 km/hr
4. 15 km/hr

Correct answer: 45 km/hr

Solution

The two slow cars are 5 km apart; the oncoming car closes this gap at relative speed (v+30) km/h. Meeting them 4 min (=1/15 h) apart gives 5/(v+30) = 1/15, so v+30 = 75, v = 45 km/h.

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