Chocolate and mint sweets are in the ratio 5:3. Fifteen chocolates are eaten, nine mint sweets are added, and the ratio becomes 2:3. Find the number of sweets at the start.
This is the shape of question that used to be worth five marks, and in Kydon’s words it is as hard as primary school ratio gets — “you cannot get harder than this, really.”
The whole difficulty sits in one step. Both sides of the ratio have changed, so before you can do anything you have to MAKE THE RATIO THE SAME. Multiply the first ratio by 3 and the second by 2, apply the multiplier to every term, and the question collapses into 15u – 45 = 6u + 18. From there it is number bonds and a model — the same two steps every ratio question ends in.
The part worth telling your child: that method has a name in secondary school. It is algebra. The model they are drawing in Primary 6 is the same solving they will do with letters in Secondary 1 — they are just not calling it that yet.
Full 9-minute lesson: https://youtu.be/JlNor59hMHo
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