Scale a recipe together. Choose a recipe and pick a new number of people. First find how much one person gets. Then find the factor, which is the new number shared by the old one. Multiply every line by it.

Before chapter 2. Beto wants to add 192 to every line, because 200 is 192 more than 8. Will that work?

The answer. No. Adding the same amount to every line does not keep the recipe the same. The soup would get 195 onions. Scaling means multiplying: 200 ÷ 8 = 25, so every line is 25 times bigger (75 onions, 50 litres of stock). When 50 litres will not fit in a 20-litre pot, the kitchen splits the guests into four sittings of 50, with 12.5 litres in each pot.

The recipe: vegetable soup (serves 8). 2 litres of stock, 3 onions, 4 carrots, 2 potatoes, a handful of noodles, and salt to taste. Chop the vegetables, soften them, add the stock, and simmer for 25 minutes. A grown-up handles the stove and the knife.

The grown-up layer. Adding instead of multiplying is one of the most common mistakes in ratio. RatioRealm's Scale and Unit work on exactly this.

For the ride home. Ask: a recipe for 4 people uses 6 eggs. How many for 10?