Hi Arif ,
If you are going to be changing any one or more of the 3 variables ( Discount , Sales Tax , Excise Duty ) often , then it is better to go with Khurrum's formula , which makes it easy to change any one of these.
If not , then the same can be further reduced to :
=0.8568 * Original Selling Price
where the factor 0.8568 is the result of offering a 20 % Discount , then adding 5 % Sales Tax , and then adding 2 % Excise Duty.
Narayan