[SOLVED] Find Base Total Amount

I believe this to be algebra problem that I have here, but I'm not positive. I'm not even sure that I can find this value mathematically, but I'm sure someone on this forum could tell me. Here is what I need to know:

If I have a base amount of $30 and a final amount of $30.70. I need to know how much of the base amount has been paid for and how much of the .70 tax was paid for.

Example:

$10 is taxable at 7%

$20 is NOT taxable

------------------

$30 Base Total

If I have this equation here:

$30.70 = $20 + ($10+($10 * 7%))

An individual only pays for $10.50.

How do I find how much of the Base amount is left?

$30 - (Portion of payment that is base amount)=Base Amount Left

How do I find how much of the tax total is left?

.70 - (Portion f paymen that is tax)=Tax Amount Left