Why do this problem?
acts as a good introduction to trianngular numbers and their patterns. It certainly is suitable for the younger pupils.
It would be best to introduce this in a totally practical way with the children observing a small triangular tower built out of cans or whatever is available. Then, you could show them a similar triangular tower that's taller and invite the pupils to talk about what they see. If their conversations don't naturally bring out the numbers of cans used, then you can ask appropriate
You could then introduce the problem orally to the class and encourage them to work in pairs or small groups.
Tell me about the numbers you can see.
What do you notice going on in these triangular patterns?
Children could try Man Food
as a follow-up activity.
Some pupils will need to go very slowly at first with the smallest of triangular arrangements and will probably benefit by having arrangements there in front of them for comparison and exploration. They could use any concrete objects to represent the cans.