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.

Let the distance from camp to explosion be d metres. The first drone takes d/30 seconds to fly to the spot, and the picture transmission is instantaneous. Time from sensor receiving sound to first picture = d/30 = 3 minutes × ... Actually, between the two pictures (5:02 PM and 5:05 PM), the time gap is 3 minutes = 180 seconds. The second drone takes d/30 seconds to fly. So d/30 = 180, giving d = 5400 m. Time for sound to reach camp = 5400/300 = 18 seconds. Time for first drone to fly = 5400/30 = 180 s = 3 minutes. So the explosion occurred 18 seconds (sound travel) + 180 seconds (drone travel) = 198 seconds before 5:02 PM. 5:02:00 PM minus 3 minutes 18 seconds = 4:58:42 PM.