In order to find how many times they will blink at the same time we need to find the common multiple of 4 and 6 between 1 and 60.
The first bulb blinks after every 4 seconds, means it blinks in multiple of 4.
The multiples of 4 = 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60.
The second bulb blinks after every 6 seconds, means it blinks in multiple of 6.
The multiples of 6 = 6, 12, 18, 24, 30, 36, 42, 48, 54, 60.
The common multiple of 4 and 6 = 12, 24, 36, 48, 60.
∴
in sixty seconds they will blink 5 times at the same time.
