if you have a price (p) and sales tax (t%). To find the total price you multiply p by (t%/100) to find the amount the tax is going to add to p than add that to p.
ex) 35 dollar item with a 4% sales tax.
35x0.04=1.4
35+1.4=36.5
36.5 is price of item plus sales tax
I hope this helps you