I need to figure reverse tax from a total

Let 'x' represent the cost before tax.

If the tax on something that costs x dollars is 6%, then the amount of tax on x is:

(6%/100%)x = (0.06)x

Since the total cost after tax is $106, then

cost before tax + cost of tax = 106

That is,

x + 0.06x = 106

Combining like terms, we arrive at the following:

1.06x = 106

Divide both sides of this equation by 1.06 to solve for x:

1.06x/1.06 = 106/1.06

x = 100

Thus, the cost before tax is $100.

You can check this be finding the cost of a 6% tax on something that cost $100, doing so by the following:

100 · (6%/100%) = 100 · 0.06 = 6

So a tax of 6% on $100 is $6, which means that the total cost is the cost of the object before tax plus the tax amount. That is,

$100 + $6 = $106