Instead of normal weighing scale a shopkeeper used forged scale. He used 1.4 kg scale while buying and 840g scale while selling, what will his overall profit percentage, if in the end he offers 10% discount?

When buying, the shopkeeper gets 1.4 kg for the price of 1 kg, so his cost per kg is 1/1.4 = 5/7 of the normal price. When selling, he gives only 840g (0.84 kg) for the price of 1 kg, so he effectively charges 1/0.84 = 25/21 times the normal price per kg. After a 10% discount, the selling price becomes 0.9 × 25/21 = 15/14. Profit percentage = (SP - CP)/CP × 100 = ((15/14 - 5/7)/(5/7)) × 100 = (5/14)/(5/7) × 100 = 50%.