From the question, we have
For the dealer in Mumbai (inter-state transaction)
CP = Rs. 20,000
IGST = 12% of Rs. 20,000 = 12/100 x 20,000 = Rs. 2400
Given, Profit = Rs. 5000
SP = Rs. 25,000
Now, for the dealer in Pune (intra-state transaction)
CP = Rs. 25,000
CGST = 6% of 25,000 = Rs. 1500
SGST = 6% of 25,000 = Rs. 1500
Thus, the GST payable by the end user at Pune is (1500 + 1500)Rs = Rs. 3000