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