Calculate sales tax Rate U.S./Canada in Excel: If pre-tax amount, sales tax amount paid, grand total paid, but UNKNOWN Sales Tax Rate?

Question

1. I need to create a calculation for Excel or on paper to (not a website or app): Partial refunds and sales tax to apply without knowing the U.S. State’s Sales Tax Rate.

Example of problem: If I know my subtotal is: $\$27.99$ And my Tax Amount is: $\$1.96$ How can I know that the Tax RATE is: $7.00\%$ ?

Excel: How do I express this in a formula in Excel allowing for manual entry of field 1 (pre-tax amt), field 2 (tax amt paid) = Rate

2. For Canadian refunds need for Excel formula: GST, PST (Federal Rate 8%, Provincial Rate 5%) are needed some times together, a calculation with IF statement (if GST, then 8%, IF PST 5%, IF GST AND PST then 13%).

Research: Website has a calculator tool that is an example:

1. http://www.csgnetwork.com/salestaxpercentcalc.html
2. http://www.calculconversion.com/sales-tax-calculator-gst-qst.html