The price of sugar rises from ₹40 per kg to ₹50 per kg. By what percent must a family cut its consumption so that its spending on sugar stays the same?

Spending = price × quantity, so the quantity must fall by the factor 40/50 = 0.8, a cut of 20%. (The price rose by 25%, but the cut is measured on the consumption, whose new base pairs with the higher price.)