Can anybody please clear the concept of TT/BILL Buying/selling Where to add margin In murgan sir s

Can anybody please clear the concept of TT/BILL Buying/selling Where to add margin In murgan sir sample paper somewhere they calculated buying rate by (spot rate- margin) + premium or - discount and some where they calculated bill buying rate by (spot rate + premium) - margin. Completely confused what to do and where to implement buying or where selling rate. Please help

 

While purchase of forex we subtract margin but for selling we add margin.

 

TT buying is aplicable where other currency comes in inr or TT selling is aplicable where inr transfered to other currency .