The price of cooking oil rises by 10%. By what percentage must a family reduce its consumption so that its spending on cooking oil stays the same?

💡 Solution & Explanation

Spending = price × quantity. Take the old price as 100 and the old quantity as 100, so the spending is 10,000. The new price is 110, so the quantity must become 10,000 ÷ 110 = 90.91. That is a cut of 9.09%. Shortcut: reduction % = rise ÷ (100 + rise) × 100 = 10 ÷ 110 × 100. Cutting by 10% would leave the family spending less than before.