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 exp…

Quantitative Aptitude ·Previously asked in UPSC Civil Services Examination 2026

View the full solved paper: GS Paper II

Question

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.

  1. A. 4:59:00 PM
  2. B. 4:59:02 PM
  3. C. 4:58:42 PM (Correct answer)
  4. D. 4:56:32 PM

Correct Answer

Option C — 4:58:42 PM

Detailed Solution & Explanation

The correct answer is Option C (4:58:42 PM).

Key Points

  • Let the distance be d. Second drone's flight (d/30) took the gap between the two pictures: 5:05 − 5:02 = 180 s, so d/30 = 180 ⇒ d = 5400 m.
  • First picture time = explosion + sound travel (d/300 = 18 s) + first drone flight (d/30 = 180 s). So explosion = 5:02 − 198 s = 4:58:42 PM.

Additional Information

  • Sound reaches the camp first (300 m/s), then the drone flies out (30 m/s); separating these delays gives Option C.