A 40-inch TV costs $320 at the store. The store advertises 20%
off all items in the store. The
sales tax is 6%. What is the sale
price including tax?

Tell me how to do it !!!

First you need to get the tax information, which commonly is based to the total cost, not the total cost with the 20% off.
To get that 6% tax you need to calculate de percentage. Which you can do by using the 3 rule *im attaching a picture below*. With the 3 rule you need to cross multiply the 6% with the 320 dlls and then divide it by 100%. This is to get the equivalents of the 100% with the 6%.
With this we calculated that the tax would be equal to 19.2 dlls.
Then, we need to calculate the 20% off. Again, using the percentage. A 100% is 1 whole, while 20% would be .2 compared to the 1. So we multiply the 320 dlls (which is the whole= 1), by the .2 (which would be equal to 20%). 320dllsx .2= 64 dlls.
Having the total of the 20% off, we need to Substract that with the total.
20% off= 64 dlls
100%= 320
320-64= 256dlls (aka 80%)
finally we need to add the tax
6% tax= 19.2 dlls
40 inch tv with the 20% off= 256 dlls
256 dlls + 19.2 dlls= the total cost with tax
which would end up being 275.2 dlls