Answer:
$20
Explanation:
You want to know the pre-tax price if the price with 5% tax added is $21.
Tax
The added tax is 5% of the original amount (x), so the amount with tax added is ...
$21 = x + 0.05x = 1.05x
$21/1.05 = x = $20 . . . . . divide by the coefficient of x
The cost before tax was added was $20.