The price of petrol goes up by 25%. Sameer raises his monthly spending on petrol by only 10%. By what percent must his consumption fall?

Spending = price × quantity, so the quantity factor is 1.10/1.25 = 0.88 — a fall of 12%. (Subtracting 25 − 10 = 15 treats the two percentages as if they shared a base; 20% is the cut he would need if his spending stayed unchanged.)