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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 watches a third light. He sees it flash at the same time as the other two, then it flashes every 7th second. How many minutes will it be before this light flashes again at exactly the same time as the other two?
This problem is a good way of introducing children to common multiples and it is also a useful context for looking at different recording methods.
As a starter, you could split the class into two groups. One group will clap every 3 beats and the other every 6 beats, while you count the beats. Ask them to predict on which beats they will all be clapping. Try other rhythms in the same way e.g. 3 and 4. Can they explain why everyone will be clapping on certain beats? How would they work out which beats these were without clapping?
In the plenary, you can specifically introduce the vocabulary of common multiples if you haven't done so already.
Music to My Ears would be a good problem for children to try next as it places greater emphasis on predicting when common multiples occur.
Some learners might find Clapping Times a good problem to try before this one.
Place four pebbles on the sand in the form of a square. Keep adding as few pebbles as necessary to double the area. How many extra pebbles are added each time?
Investigate the different shaped bracelets you could make from 18 different spherical beads. How do they compare if you use 24 beads?
How many different shaped boxes can you design for 36 sweets in one layer? Can you arrange the sweets so that no sweets of the same colour are next to each other in any direction?