Let the cost price of 1 kg = ₹100 (assume for simplicity)
He claims to sell 1 kg at cost price, but actually gives only 82% of 1 kg.
Cost of 0.82 kg
CP of 0.82 kg=100×0.82=82
Selling price
He charges for 1 kg ⇒ ₹100
Profit
Profit=100−82=18
Profit %
8218×100=821800=21.9512…
Convert to fraction:
=41900=214139%