A crockery dealer prices every plate 30% above what it costs him, but under a running scheme he hands over one plate free with every ten a customer pays for. His profit percent on such a sale is:

Let a plate cost him ₹100. To complete one scheme sale he parts with 11 plates, which cost him ₹1100, and he collects $10\times 130$ = ₹1300. Profit = ₹200 on an outlay of ₹1100 = 18.18%. (The 30% mark-up survives only if nothing is given away; treating the free plate as a flat 10% off and taking 30 − 10 = 20% mixes two different bases, and 1/11 = 9.09% is the share of the goods given away, not the profit.)