This one should be easy... just need help with interest formula.

[Jake hollcroft]

I created the "banking program" below, I just need help calculating additional interest. Interest is automatically added into the balance if they choose over 1000 deposit and more than 12 months, but I am trying to add a while loop that allows them to add additional interest if they want it. Everything works, I just don't know the right formula for the additional interest. Please look below and tell me where I went wrong.
Thank You! Line 51 is what I am having problems with, it shows the new interest, but the total is wrong when I test it.

#include <iostream>
using namespace std;

int main ()
{
	int duration;  
	double interest;
	double amount;
	char account;
	char answer;  
	int additional;

	do
	{
	cout << "How much will you like to deposit? >> ";
	cin >> amount;
	cout << endl;
	cout << "What's your preferred account type"
		 << "\nEnter S for saving or C for checking >> ";
	cin >> account;
	cout << endl;

	switch (account)
	{
	case 'c':
	case 'C':
		interest =  amount  * 0.05;
		break;
	case 's':
	case 'S':
		interest = amount * 0.10;
		break;
	default:
		cout << "\nInvalid account type. Try again!" << endl;
		cin.get();
		cin.get();
		return 0;
	}

	cout << "Enter the time duration in months >> ";
	cin >> duration;
	cout << endl;

	if (duration >= 12 && amount >= 1000)
		cout << "Amount Due = $" << amount + interest 
		     << endl;
	else
		cout << "Amount Due = $" << amount << endl;
	cout<<"How much additional interest would you like?"<<endl;
	cin>>additional;
	cout<<"Your new interest is>>"<<additional+amount+interest*amount/100;

	cout << "\n\nDo you wish to perform another operation"
		 << "\nEnter y for yes or any key to exit >> ";
	cin >> answer;
	cout << endl;
	}
	while (answer == 'y' || answer == 'Y');

	cout << "\nProgram terminating..... bye !!!" << endl;

	return 0;
}

[Shren]

cout<<"Your new interest is>>"<<additional+amount+interest*amount/100;


If you are adding additional interest, shouldnt it be adding it to the prevous interested?

amount + (interest+additional)*amount/100 ?

[AeonFlux1212]

amount should be multiplied by 1.05 and 1.10 respectively when assigned to interest... and then you don't need to add the interest, because they're already included in the amount variable...

[AeonFlux1212]

Can you explain what does "additionnal interest" refers too in the real world?