A shopkeeper's purchase record reads — Rice: 12 kg for ₹720. Wheat: 20 kg for ₹640. Oats: 8 kg for ₹800. What is the average cost per kilogram of all the grain he bought, in ₹?

Total cost = 720 + 640 + 800 = ₹2160 for 12 + 20 + 8 = 40 kg, so the average is 2160 ÷ 40 = ₹54 per kg. Averaging the three unit prices ₹60, ₹32 and ₹100 as (60 + 32 + 100)/3 = ₹64 is wrong because unequal quantities were bought.