These are the Paper 2 questions where nobody hands you the equation — you build it. The method is always the same three moves. Choose the quantity you know least about and call it a letter. Write every other quantity in terms of that letter. Then find the sentence that says two things are equal — a total, a perimeter, an amount left over — and turn it into an equation. Solve it, and read the question again to check which number it actually asked for: the letter is often not the final answer.
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