A shopkeeper buys an article whose list price is ₹8000 at some rate of discount from the wholesaler. He sells the article to a consumer at the list price. The sales are intra-state and the rate of GST is 18%. If the shopkeeper pays a tax (under GST) of ₹72 to the State Government, find the rate of discount at which he bought the article from the wholesaler.

Given:
List Price of the Article = ₹8000
Let discount rate be $x$%

Cost price of article for shopkeeper = List Price - Discount

$= 8000 - \Big(\dfrac{x}{100}\times8000\Big) \\[0.5em] = 8000 - 80x$

As the sales are intra-state and the rate of GST is 18%, so GST comprises of 9% as CGST and 9% as SGST. wholesaler sells the article to shopkeeper for $8000 - 80x$, amount of GST collected by wholesaler from shopkeeper(or paid by shopkeeper)

CGST = $9$% of $(8000 - 80x)$ = $\dfrac{9}{100}\times(8000 - 80x)$ = $(720 - 7.2x)$

SGST = $9$% of $(8000 - 80x)$ = $\dfrac{9}{100}\times(8000 - 80x)$ = $(720 - 7.2x)$

∴ Amount of Input GST of shopkeeper:

Input CGST = $720 - 7.2x$
Input SGST = $720 - 7.2x$

Cost Price for Consumer = List Price of the article (as given in question) = 8000

The amount of GST collected by shopkeeper (or paid by consumer):

CGST = 9% of 8000 = $\dfrac{9}{100}$ x 8000 = ₹720

SGST = 9% of 8000 = $\dfrac{9}{100}$ x 8000 = ₹720

∴ Amount of Output GST of shopkeeper:

Output CGST = ₹720
Output SGST = ₹720

As the shopkeeper pays a tax (under GST) of ₹72 to the State Government:

Tax paid to state government by shopkeeper = Output SGST of shopkeeper - Input SGST of shopkeeper = ₹72

$\Rightarrow 720 - (720 - 7.2x) = 72. \\[0.5em] \Rightarrow 7.2x = 72 \\[0.5em] \Rightarrow x=\dfrac{72}{7.2} =\bold{10}\%$