From the question,
For the dealer A (intra-state transaction)
SP = Rs. 8,000
For the dealer B (intra-state transaction)
CP = Rs. 8,000
CGST = 9% of 8,000 = Rs. 720
SGST = 9% of 8,000 = Rs. 720
Given profit = Rs. 1,200
SP = Rs. 9,200
For the dealer C (inter-state transaction)
CP = Rs. 9,200
IGST = 18 % of Rs. 9,200 = 18/100 x 9200 = Rs 1656
Input Tax = Rs. 1,656
As the dealer in Patna does not sell the product.
Thus, the output GST (tax on sale) = Rs. 0