Based on the number of seconds it takes for each light to blink, they will both blink at 20 seconds.
The green light blinks every 4 seconds and the yellow, every 5 seconds.
The time that they will blink together depends on the lowest common multiple of both numbers.
Multiples of 4 = 4, 8, 12, 16, 20
Multiples of 5 = 5, 10, 15, 20
The lowest common multiple is 20 which means they will blink together after 20 seconds.
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