Please, please help with this math question!?

Question

This question is bugging me to death. Please explain as best as you can, thank you.

A neon sign display has three sets of flashing lights. At 1:00, all the lights are off. Set A remains on for 15 seconds, then off for 1 second. Set B remains on for 20 seconds, then off for 1 second. Set C remains on for 6 seconds then off for 1 second. How many seconds after 1:00 will all 3 sets be off again?

Thank you for your time!

Answer 1

15 + 1 + 20 + 1 + 6 =

idk your answer is as good as mine

have a great 2008

Answer 2

at 1:00 + LCM(16, 21, 7) sec = at 1:05:36 i.e. after 336 sec.