This isn't one of those "quick mental tricks" as offered in my Hub on how to work out percentages, but here's how you'd figure out either price with sales tax or discounts:
For sales tax, if you know what the rate of sales tax is, you would multiply the the price by 100 % plus the rate of sales tax. For example, with a tax of 6%, you'd calculate the pre-sales-tax price by 106%
A quick way to do it may be to move the decimal point over 2 places to get 1%, multiply whatever 1% is by 6 (if that's the rate, for example), and then add that 6% to the pre-sales-tax price.
A discount: If you know there's something like a 5% discount, you would know that you'd be paying 95 percent of the price. You could either calculate 95% of the price; or else you could move the decimal point over one place to get 10%, think of half of that to get 5%, and then subtract that 5% (either in your head, on paper, or with a calculator) from the price..
Another way, move the decimal point over 2 places to get 1% of the pre-discount price. Multiply that 1% by 5 to get 5% - and then subtract the 5% from the price.