Why do 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
Then you could introduce the flashing lights context and ask children to work in pairs on it. After a short time, stop them briefly to share some of the different ways they are working and, in particular, to look at what they are writing down to help them. For example, some might list multiples, some might list consecutive numbers but highlight multiples in some way, some might colour
numbers in the $100$ square ... You could talk about the advantages of each method discussed. In this instance, the recording is only for them. What might they do differently if they were recording their work for someone else to understand?
In the plenary, you can specifically introduce the vocabulary of common multiples if you haven't done so already.
When will the first light flash?
When will the second light flash?
So when will they flash together?
What do you notice about the times when they flash together?
How would you predict when they will flash together next?
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.