Let the reduction given by shopkeeper be ₹ x.
Reduced price of the article = ₹(16800 - x)
Clearly, the reduced price of the article + GST on it should be = ₹18000
⇒ ₹(16800 - x) + 18% of ₹(16800 - x) = ₹18000
On solving we get
16800−x+3024−18x100=18000
⇒ 118x100=₹19824−₹18000=₹1824
⇒x=₹1545.76
Hence, reduction given by the shopkeeper = ₹1545.76
Alternate method :
Let the reduced price be ₹x
Reduced Price + GST on it = ₹18000
x + 18% of x = ₹18000
⇒ 118x100=₹18000
⇒x=₹15254.24
Reduction given by the Shopkeeper
= ₹16800 - ₹15254.24
= ₹1545.76