A and B toll together every LCM(24, 60)= 120 seconds. There are 3600 seconds in one hour and 120 divides into 3600 30 times. A and B toll together (perhaps with C) 30 times between 1100 and 1200 hours. A and C toll together every LCM(24, 90)= 360 seconds and 360 divides into 36000 100 times. A and C toll together (perhaps with B) 100 times between 1100 and 1200. B and C toll together every LCM(60, 90)= 180 seconds and 180 divides into 3600 20 times. B and C toll together (perhaps with A) 20 times between 1100 and 1200.

Soat leasttwo bells toll together 30+ 100+ 20= 150 times. You say you have already found the answer to (a), the number of times all three bells toll together. Since those times will be counted in each of the three computations above, subtract3times that number from 150 to find the number of times exactly two bells toll together.