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

To find when the three bells will toll together again, we need to calculate the Least Common Multiple (LCM) of their intervals, which are 9, 12, and 15 minutes. The prime factorization of the numbers is: 9 = 3 x 3 12 = 2 x 2 x 3 15 = 3 x 5 The LCM is 2 x 2 x 3 x 3 x 5, which equals 180 minutes. Since 60 minutes make an hour, 180 minutes is equal to 3 hours. If the bells first tolled together at 8.00 a.m., they will toll together again 3 hours later. 8.00 a.m. plus 3 hours is 11.00 a.m. Therefore, option C is the correct answer.