Regular price: $419
Sale price: (1-0.205)($419) = $333.11
The bike will cost $333.11 before tax. You haven't shared the tax rate, but I can at least tell you that the cost of the bike after tax will be
(1.00 + rate as a decimal fraction)*$333.11.
Example: If the tax rate is 7.5%, then the bike will cost (1.00+0.075)*($333.11).
