Four different electronic devices make a beep after every 30 minutes, 1 hour, \(3 / 2\) hour and 1 hour 45 minutes respectively. All the devices beeped together at 12 noon. They will again beep together at ____________
A. 12 midnight B. 3 am C. 6 am D. 9 am
Correct Answer: D
Explanation
Four different devices beep after every 30 mins, 60 mins, 90 mins and 105 mins. LCM of \(30,60,90\) and 105 is 1260. Which means the devices beep together after every 1260 mins \(=1260 / 60=21\) hours Hence 12 noon \(+21\) hours \(=9\) am
