Question

On the coast there are three lighthouses.
The first light shines for 3 seconds, then is
off for 3 seconds.
The second light shines for 4 seconds, then is
off for 4 seconds.
The third light shines for 5 seconds, then is
off for 5 seconds.
All three lights have just come on together.
When is the first time that all three lights will be off? please help

5 years ago