Getting Mathematical Errors Trying to calculate base price of gas

I am trying to solve a problem that requires me to write code to find the base price of a gallon of gas. I am provided with the cost of a gallon of gas, the amount of federal and state tax, and the percent for sales tax. I've tried formatting the equation a couple different ways and the math is always wrong. This is the closest I've gotten (the output is $3.289 which is only .073 cents off, which when I do the calculations by hand happens to be the amount of sales tax so it is as if it is ignoring the end of the equation completely). Could I please get some assistance? I've included the entire problem I was given below.

double pumpPrice=3.89, fedTax=.184, stateTax=.417, salesTax=2.25;
double basePrice, totalTax, percentTax, profit_bigOil ;

basePrice = pumpPrice-fedTax-stateTax-((salesTax*basePrice)/100) ;

cout <<"The base price for a gallon of gas is $" <<basePrice << endl;


Create a program, i.e. a Netbeans project with the following inputs

0) Take your pump price, whatever you paid at the pump along with taxes and calculate percentage that you pay in taxes.

Pump Price = Base Price + Fed Tax + State Tax + Base Price * Sales Tax


1) Oil Company Profit -> https://www.forbes.com/2011/05/10/oil-company-earnings.html#6c3f9a9f2dc8

7cents/gallon to 6.5%/gallon


2) Taxes -> http://www.latimes.com/politics/essential/la-pol-ca-essential-politics-updates-california-s-increased-gas-tax-goes-into-1509552219-htmlstory.html

Fed Tax -> 18.4 cents/gallon State Tax -> 41.7 cents/gallon State Sales Tax -> 2.25%


Calculate the following

1) The base price for a gallon of gas = $

2) The total tax on a gallon of gas = $

3) Percentage price due to gas tax = %

4) Oil Company profit range = %

basePrice = ...

basePrice

basePrice

basePrice + basePrice * salesTaxFraction

basePrice * (1 + salesTaxFraction)

basePrice

1 Answer
1

basePrice = pumpPrice-fedTax-stateTax-((salesTax*basePrice)/100) ;
// ^^^^^^^^^ ^^^^^^^^^


That statement is attempting to set basePrice so it's probably a bad idea to use the uninitialised basePrice in the calculation.

basePrice

basePrice

Methinks you need to work out the formula you need to use before translating that into code. A good start would be to take the base price and document how each of the taxes is applied to it to get the pump price. Then it's a simple matter of reversing that process.

For example (and I have no idea if this is actually how you US bods do it with your overly complex tax system), let's say that the base price has sales tax of 2.25% applied, and then the absolute values of state and federal taxes added. That would mean:

pumpPrice = basePrice * (1 + salesTax / 100) + stateTax + federalTax


The reverse of that (working out base price from pump price) would come from:

pumpPrice = basePrice * (1 + salesTax / 100) + stateTax + federalTax
=> pumpPrice - federalTax = basePrice * (1 + salesTax / 100) + stateTax [*a]
=> pumpPrice - federalTax - stateTax = basePrice * (1 + salesTax / 100) [*b]
=> (pumpPrice - federalTax - stateTax) / (1 + salesTax / 100) = basePrice [*c]


^[*a] Subtract federal tax from both sides.
^[*b] Subtract state tax from both sides.
^[*c] Divide both sides by sales tax (adjusted) rate.

And, voila, swap the sides and you have your formula:

basePrice = (pumpPrice - federalTax - stateTax) / (1 + salesTax / 100)


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