Introducing algebra

Sam sets up displays of cat food in his shop in triangular stacks. If Felix buys some, then how can Sam arrange the remaining cans in triangular stacks?

We can arrange dots in a similar way to the 5 on a dice and they usually sit quite well into a rectangular shape. How many altogether in this 3 by 5? What happens for other sizes?

Take a counter and surround it by a ring of other counters that MUST touch two others. How many are needed?

Can you find pairs of differently sized windows that cost the same?