I have a small grocery store. I have a bag of money containing $325. This represents the mornings receipts and a tax of 5.75% is included in the total. How can I separate the gross take from the tax take so that I'll know how much is mine and how much is the state's?

Think about how you would calculate the tax if you knew the gross take. Suppose that your gross take was $G then to find the tax owing you would multiply $G by 5.75%, that is the tax owing would be 0.0575xG dollars. Hence the bag would contain G + 0.0575xG or 1.0575xG dollars.

In your particular situation you have 1.0575xG = 375 so G = 375/1.0575 = $307.33