A trader cheats both his supplier and customer by using faul

[BTGmoderatorAT]

A trader cheats both his supplier and customer by using faulty weights. When he buys from the supplier, he takes 10% more than the indicated weight. When he sells to his customer, he gives the customer a weight such that 10% of that is added to the weight, the weight claimed by the trader is obtained. If he charges the cost price of the weight that he claims, find his profit percentage.

A. 20%
B. 21%
C. 21.33%
D. 22.109%
E. 23%

I'm confused how to set up the formulas here. Can any experts help?

[GMATGuruNY]

A trader cheats both his supplier and customer by using faulty weights. When he buys from the supplier, he takes 10% more than the indicated weight. When he sells to his customer, he gives the customer a weight such that 10% of that is added to the weight, the weight claimed by the trader is obtained. If he charges the cost price of the weight that he claims, find his profit percentage.

A. 20%
B. 21%
C. 21.33%
D. 22.109%
E. 23%

Let the cost price per pound = $1.

Cost:
Let the trader pay his supplier for 100 pounds of product.
Since the cost price = $1 per pound, the total cost for 100 pounds of product = 1*100 = $100.
Since the trader cheats and takes 10% extra, the weight received = (100 pounds) + (10% of 100 pounds) = 100 + 10 = 110 pounds of product.
Thus, 110 pounds of product is purchased for a total cost of $100.

Revenue:
When he sells 110 pounds of product to his customer, the trader adds 10% to the weight, with the result that the weight charged to the customer = (110 pounds) + (10% of 110) = 110 + 11 = 121 pounds of product.
Since the selling price per pound is the same as the cost price per pound ($1), the total revenue received from the customer = 1*121 = $121.

Profit:
(total revenue) - (total cost) = 121 - 100 = 21.
Profit percentage:
profit/(total cost) = 21/100 = 21%.

The correct answer is B.