A retailer buys books from a publisher at a discount of 30% on the printed price and sells them to customers at the full printed price. His profit percent is:

Take the printed price as ₹100. The retailer's cost is ₹70 and his selling price ₹100, so his profit is ₹30 on a cost of ₹$70=30/70\times 100=42.86$%. (Quoting the 30% discount as the profit uses the printed price as the base; profit percent is always measured on cost. Dividing 30 by 130 gives the other trap, 23.08%.)