GST Payment

purchase price is 45000/- inclusive of gst 

and sales price inclusive of gst that is 50000/-

in other words

cost + gst = final price (consider both sales and purchase )

here formula is final price sales or purchase what is given 

for purchase it is 45000/128*100 =35156.25 is cost price 

for purchase add gst on it @ 28% u will get figure 

9,843.75 now add it to 35156.25+9843.75 = 45000

now u have  the fugire with u 

apply this with sales

now part of how much gst u pay

as per rules 

payable gst (sales) - receivable gst (purchase)

u will get ur answer how much u r going to pay