Ratio

1) We can see that the ratio of chocolate to cornflakes is 20:15

But this can be simplified further. The Greatest Common Factor is 5, so:

If we have 20 parts chocolate to 15 parts cornflake, divide each side by 5 and we get 4:3

This is the ratio of chocolate to cornflakes.

2) We need to work out how much chocolate and cornflakes we need to make 21 cakes.

The recipe is for 1 cake but we want 21, so multiply both ingredients by (21 cakes / 1 cake), or simply by 21:
 * 21×20 = 420
 * 21×15 = 315

Question: If we only have 400g of chocolate, how many cakes can we make, and what quantity of cornflakes do we need?