Estimate the sales tax

I have a problem. If I get a receipt from a purchase all it lists is the item price but I don't see the sales tax rate which adds the amount to get the grand total. Is there an easy way to estimate the sales tax rate and create a function to automatically estimate it's percentage if you have the grand price total?

The first row lists the column headers

A2 is the item price
B2 is the unknown sales tax rate
C2 is the grand total price of the item plus tax rate.

If I have the item price and the grand total price...can a function be made to estimate the sales tax? Or put another way...if I have the initial sales price but don't have the sales tax rate but can't find the grand total price unless I have the sales tax...is there a way to generate a function to automatically estimate what the total cost would be if a sales tax were included?

Thanks in advance.