question_answer

Four electronic devices make a beep after every 30 min, 1 h, \[1\frac{1}{2}\text{h}\] and 1 h 45 min respectively. All the devices beeped together at 12 noon. At what time will they beep together again?

A) 6 am

B)  9 pm

C)  9 am   

D)  7 pm

Correct Answer: C

Solution :

Intervals of beeping 30 min, 60 min, 90 min, 105 min. Interval of beeping together =LCM of 30 min, 60 min, 90 min, 105 min \[=(3\times 5\times 2\times 2\times 3\times 7)\] =1260 min = 21 h.                             So, they will beep together again next morning at 9 am.