Sale amount 399.79 sale tax 5%=> 399.79 dollars is the 100% value and the amount of the sales .Now, it is being charged with a 5% sales tax.What we need to do first is to solve for the value of 5% tax, then add it to the amount of the sales.Let's start solving this equation:=> 5% = 5% / 100% = .05=> 399.79 * .05 = 19.99Now, let's add=> 399.79 + 19. 99 = 419.78 dollars will be the new price of the sales.