If an item is advertised at 70 percent off the full price, it retails at a price that is three-tenths of the original sticker price, before sales tax. Sales taxes are added after this calculation is complete to get the price to pay at the register.
The easiest way to calculate what the discounted item costs before tax is to take the original price and multiply it by 0.3, which calculates 30 percent of the first amount. This is equivalent to calculating 100 percent of regular price minus 70 percent of regular price.
Online tools are also available to help with this sort of calculation, such as PercentOffCalculator.com.