If the discount is 20%, then the customer pays the remaining 80%
We'll involve the multiplier 0.80
We'll also involve 1.06 to indicate a 6% increase, due to the sales tax.
Overall, the combined multiplier is 0.80*1.06 = 0.848
So in reality, the true discount rate is 1-0.848 = 0.152 = 15.2%
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In the problem you posted, you didn't mention what the original TV price was. So in its current form, I cannot answer the question fully.
However, let's say the original price was $1000
The sale price would then be 0.848*1000 = 848 dollars
This is of course a hypothetical example.