• # question_answer Four different electronic devices make a beep after every 30 minutes, 1 hour, $1\frac{1}{2}$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) 3a.m.                          C)  6a.m.             D)         9a.m.

L.C.M. $(30,\,\,60,\,\,90,\,105)={{2}^{2}}\times {{3}^{2}}\times 5\times 7$' = 1260 mins = 21 hours