Question

Three bells toll at intervals of 9, 12 and 15 minutes respectively. All the three begin to toll at 8 a.m. At what time will they toll together again?  

(a) 8.45 a.m. (b) 10. 30 a.m. (c) 11.00 a.m. (d) 1. 30 p.m.

Answer 1

(c) Bells will toll together again at a time, which is obtained by taking L.C.M. of their individual tolling intervals. 

L.C.M. of 9, 12 and 15 = 180 min 

They will toll together again after 180 min, i.e. 3 hours. 

Time = 8 + 3 = 11 a.m.