Let the MP 1 kg tea be Rs. 1,
Then CP of 20 kg with discount = 20 x 0.9 = Rs.18
Also 1 kg tea is free. So the retailer gets tea worth Rs. 21 by paying Rs. 18 only.
profit % = ( goods left / goods sold) x 100
= (21-18 / 18 x 100 = 16.66 %
(Since the retailer earn Rs. 3 on each Rs.18)