The price with tax is 100% + 8% = 1.08 times the price before tax. The discount multplies that price by 100% -20% = 0.80. The discount multiplier is the same whether the discount is applied before or after the tax.
The combined effect of the two multipliers is ...
... 1.08 × 0.80 = 0.864
so the discounted price with tax added is 0.864 × $50 = $43.20
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Perhaps you're supposed to figure the tax (8% of $50 = $4), add the tax ($50 +4 = $54), then subtract the discount (20% of $54 = $10.80) to get