When the product is sold from Kanpur to Banaras (It is an intra-state transaction).
For the Dealer in Kanpur, SP = ₹1200
CGST = SGST = 9% of ₹1200 = ₹108
When the product is sold from Banaras to Ranchi(It is an Inter-state transaction).
For the Dealer in Banaras :
Input Tax Credit(ITC) = ₹108 + ₹108 = ₹216
CP = ₹1200.
Profit = ₹400
SP = CP + Profit = ₹1200 + ₹400 = ₹1600
IGST = 18% of ₹1600 = 18100×1600 = ₹288
∴ Net GST payable at Banaras = Output GST - ITC
= ₹288 - ₹216 = ₹72