You are correct that the complete on-off cycles are 6, 8, and 10 seconds long. But can you explain what you mean by"double time"? Given the result, I suspect it is what I would refer to by the initials"lcm", which is correct. But at 120 the 5-second light would have gone through 12 complete cycles, and will start its 13th cycle. By 121, it and all the others will have already been lit for 1 second.