A trader sells wheat at 20% profit and uses weight 20% less than the actual measure. His gain per cent is?

Let the market weight = 1 kg = 1000 g
Real weight = 80% of 1000 = 800 g
Let the CP of 1 g = ₹ 1
∴ CP of 800 g = ₹ 800,
CP of 1000 g = ₹ 1000
SP = 120% of 1000 = (120/100) x 1000 = ₹ 1200
Gain = 1200 - 800 = ₹ 400
∴ Gain % = (400/800) x 100% = 50%