On the coast there are 3 lighthouses. All 3 turn on at time zero.
- 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.

A distant ship has an optical sensor aimed at the set of lighthouses, but it is far enough away that the sensor only activates when two of the lights are on at the same time.
Over the course of one minute, how often is the sensor active?

Original problem: Lighthouse Crossed Three Lights Muse