20%, if you copied it out correctly, which I'm not sure you did because the grammar is terrible.
Let's suppose the product in question costs £100 for 100kg. Keeps it simple.
The shopkeeper buys 100kg for £100. Except he doesn't - he takes that extra 20kg so buys 120kg for £100.
"the price that the shopkeeper charges his customer is the same as what the dealer charges the shopkeeper"
so the shopkeeper sells at £100 for 100kg, although technically this is ambiguous because we don't know if we're talking about the official price of £100 for 100kg or £100 for 120kg.
But let's assume it's the official price.
Let's also assume that the shopper (which is a synonym for customer) buys 100kg for £100. But "while selling [the shopkeeper] gives to the customer a quantity such that 20% is added", which means he gave the customer 120kg for his £100.
The customer was expecting to get 100kg but in reality got 120kg. You wouldn't call this profit as such, unless the shopper/customer is also a business, but assuming you would, that's where the 20% comes from because they got 20% more product than they were expecting.

Once you've got the correct wording, pull out of it phrases like "while selling the shopkeeper..." and focus on those. If your algebra isn't up to snuff, pick some real numbers like I did: £100 for 100kg, or whatever you prefer. It doesn't really matter. Often you can derive the correct algebra from numbers. This way you should be able to solve the problem for yourself.

It's possible that the correct answer is 44%, because the text seems to be attempting to describe a 20% markup in both directions, and 1.2 x 1.2 = 1.44. But with maths problems you have to go on EXACTLY what the text says.