A packaging wholesaler charges ₹20 per carton for the first 100 cartons of an order and ₹16 per carton for every carton beyond that. A shop orders 250 cartons. Measured against the ₹20 list price, the shop's effective discount on the whole order is what percent?

The order costs $100\times 20+150\times 16=2000+2400=$ ₹4400, whereas 250 cartons at the list price would cost ₹5000. The saving is ₹600 on ₹5000 = 12%. (The ₹16 slab is 20% below the list price, but it applies to only 150 of the 250 cartons, so the discount across the order is smaller; ₹17.60 is the average price paid per carton, not a percentage at all.)