TITLE: Calculate Subtotal from Total and Percentage
QUESTION [5 upvotes]: If I know my tax rate, and I know the total, I'm looking for the subtotal.
So if (subtotal + (subtotal * taxrate)) = total, and I have total and taxrate, how do I get subtotal?

REPLY [6 votes]: Like many mathematicians, I prefer shorter variable symbols. So let $S$ be the subtotal, and let $T$ be total.
Let the tax rate be $r$. If for example the rate is $23\%$, then use $r=0.23$.
We have by your equation
$$S+Sr=T.$$
this can be rewritten as 
$$S(1+r)=T.$$
Divide both sides by $1+r$. We get
$$S=\frac{T}{1+r}.$$
Example: Let the tax rate be $23\%$, and let the total be $10000$. then the subtotal is $\dfrac{10000}{1.23}$. This is about $8130.08$.