Norrie is watching the aircraft warning lights on the tops of some tall buildings in the city. He sees two lights flash at the same time, then one of them flashes every 4th second, and the other flashes every 5th second.
How many times do they flash together during a whole minute?

Norrie then watched a third light. He saw it flash at the same time as the other two, then flash every 7th second. How many minutes before this light again flashes at exactly the same time as the other two?