Three grades of tea costing ₹120, ₹150 and ₹180 per kg are mixed in the ratio 2 : 3 : 5. The cost of the mixture per kg (in ₹) is:

Take 2 kg, 3 kg and 5 kg. Cost = $2\times 120+3\times 150+5\times 180=240+450+900=$ ₹1590 for 10 kg, so the mixture costs ₹159 per kg. (The plain average of the three prices, ₹150, ignores that the dearest grade makes up half the mixture.)