Copyright © University of Cambridge. All rights reserved.
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 $4$th second, and the
other flashes every $5$th 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 $7$th second. How many minutes before this light again flashes at exactly the same time as the other two?