Six bells commence tolling together at 7:59 am. They toll at intervals of 3,6,9,12 and 15 seconds respectively. How many time will they toll together till 8:16 am? (excluding the toll at 7:59 am)

6

LCM (3, 6, 9, 12, 15, 18) = 180 

So, the bells will toll together after every 180 seconds (3 minutes)

In 17 minutes, number of tolling together = 5 + 1 = 6 times