A porridge recipe makes two bowls: one cup of oats and two cups of water. But there are twenty-eight of us in the class this morning and everyone is hungry.

Roughly how much oats and water do you think we would need to feed the whole class? Have a guess before we work it out exactly.

The interactive shows a recipe scaled up to feed more people, first pancakes then soda bread. Each time, predict the new amounts before we reveal them, and notice that both bars have grown by the same factor.

This is the key one: it does not matter whether the recipe is in cups, eggs or grams — proportion always multiplies everything by the same factor.

Today we build a class-porridge recipe on the ratio bars. Our recipe for 2 people is 1 cup of oats to 2 cups of water. We will scale it together in this order: first for 4 people (×2), then for 6 people (×3), then for 14 people (×7). Before each reveal, predict the oats and the water. Watch how both bars grow by the same factor every single time.

We work through these proportion problems one at a time. For each one, predict the answer, then set the bars and check before we move on:

• First, a recipe to scale up: a scone recipe for 6 uses 3 cups of flour to 1 cup of milk. Work out the factor and scale it for 18.
• Next, a sharing problem: 2 helpers can set out 10 chairs, so how many can 6 helpers set out at the same rate?
• Last, our ceiling problem — a journey: a car travels 30 km on 2 litres of fuel, so how far will it go on 8 litres?

If we have time, try the stretch: how far would that same car go on 10 litres?