An explosion takes place at a certain distance from an army camp. As soon as the sensor in the camp receives the sound of the explosion, a drone starts flying towards the spot of explosion. The drone clicks a picture from the spot and the camp receives it at the same time. Immediately another drone starts flying to the spot and it also sends a picture as soon as it reaches the spot. The two pictures were received at 5:02 PM and 5:05 PM, respectively. If the speed of the drones is 30 m/s, at what time did the explosion take place? Assume that the speed of sound is 300 m/s.
- (a) 4:59:00 PM
- (b) 4:59:02 PM
- (c) 4:58:42 PM ✓ UPSC's answer
- (d) 4:56:32 PM
Why the answer is (c)
• Let the spot be d metres from the camp. The second drone leaves at 5:02 PM, flies d metres at 30 m/s and sends its picture at 5:05 PM, so d/30 = 180 s and d = 5400 m.
• The first drone also needed 180 s, so it left, i.e. the sensor heard the sound, at 5:02 PM − 3 min = 4:59:00 PM.
• Sound took 5400/300 = 18 s to reach the camp.
• The explosion happened at 4:59:00 − 18 s = 4:58:42 PM: (c).
Why the other options are wrong
- (a) 4:59:00 PM
- 4:59:00 PM is when the sound reached the camp, not when the explosion happened.
- (b) 4:59:02 PM
- 4:59:02 PM adds the sound delay instead of subtracting it.
- (d) 4:56:32 PM
- 4:56:32 PM subtracts far more than the 18 s the sound took.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2026, held on 24 May 2026. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.