20% off means you are paying 80% of the original price ( 100% - 20% = 80%)
Multiply the price of the tv by 80% to find the sale price:
500 x 0.80 = 400
Now multiply the sales price by 1 plus the tax rate to find final cost:
400 x 1.06 = 424
Final price including tax is $424
You could subtract the denominator fro the numerator which would give you a total sum of 1 thousand an fifty five