In , lucrezia
spake thusly:

I have an amount that represents the sales tax we have collected
for the last quarter. I need to be able to calculate what the
total sales are from this number. The tax is 6%. I know what my
total sales are, but some sales are tax exempt. I know there is
a really easy way to calculate backwards to find the total sales
that were taxed at 6%, but it seems to escape me at the moment.

Perhaps you want:

tax_amount / 0.06 = pretax_basis

DR