Multiples of 4 = 4, 8, 12, 16, 20, 24, 28, 32, 36, 40
Multiples of 5 = 5, 10, 15, 20, 25, 30, 35, 40, 45, 50
Multiples of 8 = 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120
So, common multiples of 4, 5 and 8 = 40, 80, 120,
As common multiples are going up in 40's, so we can keep counting in 40's to find more common multiples of 4, 5 and 8.
⇒ Common multiples = 40, 80, 120, 160, 200, 240
Number of seconds in 4 minutes = 4 × 60 = 240 seconds
So, after 10:00 AM three clocks will ring together for the second time after 40 seconds at 10:00:40 and again and again after every 40 seconds.
So altogether they will ring 6 times within a time span of 4 minutes from 10:00 AM.