To find a percentage of a number, multiply by the decimal form of that percentage(20%=0.20). So, take the cost, $430, and multiply by common state sales tax (7% for me)---430 x 0.07 = 30.10 The sale tax rate is 7%.
Note 1: A more general solution is for some constant C.
Note 2: It might be tempting to say the general solution is , but that is not the case because and we can see that would only be true for C = 0, so that is why does not work.