Two cities A and B are 360 km apart. A car goes from A to B with a speed of 40 km/hr and returns to A with a speed of 60 km/hr. What is the average speed of the car ?
- (a) 45 km/hr
- (b) 48 km/hr ✓ UPSC's answer
- (c) 50 km/hr
- (d) 55 km/hr
Why the answer is (b)
• The total distance traveled is the sum of the distance from A to B and B to A, which is 360 km + 360 km = 720 km.
• The time taken to travel from A to B is calculated by dividing the distance by the speed: 360 km / 40 km/hr = 9 hours.
• The time taken to return from B to A is calculated by dividing the distance by the speed: 360 km / 60 km/hr = 6 hours.
• The total time for the round trip is the sum of the individual times: 9 hours + 6 hours = 15 hours.
• The average speed is the total distance divided by the total time: 720 km / 15 hours = 48 km/hr.
• Therefore, the correct option is (b).
Why the other options are wrong
- (a) 45 km/hr
- 45 km/hr is the arithmetic mean of the two speeds, which is incorrect for average speed over equal distances.
- (c) 50 km/hr
- 50 km/hr is not the result of the total distance divided by the total time calculation.
- (d) 55 km/hr
- 55 km/hr is not the result of the total distance divided by the total time calculation.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2015, held on 23 August 2015. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.